[Xperia Arc Q&A, Help & Troubleshooting] laaaaance:
I flashed a new rom without enabling debugging mode. Now my phone starts with the Sony logo then the triangle warning sign with a picture of a phone beside it. After that, it turns off.

I tried flashing it again, here are the logs:

08/006/2016 15:06:01 - INFO - Device connected in flash mode

08/006/2016 15:06:25 - INFO - Selected Bundle for SonyEricson Xperia Arc S(LT18i). FW release : 4.1.B.0.587. Customization : Customized MEA

08/006/2016 15:06:25 - INFO - Preparing files for flashing

08/006/2016 15:06:41 - INFO - Device disconnected

08/006/2016 15:06:50 - INFO - Please connect your device into flashmode.

08/007/2016 15:07:09 - INFO - Device connected in flash mode

08/007/2016 15:07:10 - INFO - Opening device for R/W

08/007/2016 15:07:10 - INFO - Reading device information

08/007/2016 15:07:10 - INFO - Phone ready for flashmode operations.

08/007/2016 15:07:10 - INFO - Opening TA partition 2

08/007/2016 15:07:10 - INFO - Current device : LT18i - BX902VRQF2 - 1255-1481_R8D - 1253-8238_4.0.2.A.0.62 - WORLD-i_4.0.2.A.0.62

08/007/2016 15:07:10 - INFO - Closing TA partition

08/007/2016 15:07:10 - INFO - Start Flashing

08/007/2016 15:07:10 - INFO - Processing loader.sin

08/007/2016 15:07:10 - INFO - Checking header

08/007/2016 15:07:10 - INFO - Flashing data

08/007/2016 15:07:13 - INFO - Loader : S1_Loader_Root_773f - Version : R4A069 / Boot version : R9B031 / Bootloader status : UNROOTABLE

08/007/2016 15:07:13 - INFO - Max packet size set to 64K

08/007/2016 15:07:13 - INFO - USB buffer size set to 512K

08/007/2016 15:07:14 - INFO - No flash script found. Using 0.9.18 flash engine

08/007/2016 15:07:14 - INFO - Opening TA partition 2

08/007/2016 15:07:14 - INFO - Writing TA unit 00002774. Value : 01

08/007/2016 15:07:14 - INFO - Closing TA partition

08/007/2016 15:07:14 - INFO - Opening TA partition 2

08/007/2016 15:07:14 - INFO - Closing TA partition

08/007/2016 15:07:14 - INFO - Opening TA partition 2

08/007/2016 15:07:14 - INFO - Processing adsp.sin

08/007/2016 15:07:14 - INFO - Checking header

08/007/2016 15:07:14 - INFO - Flashing data

08/007/2016 15:07:17 - INFO - Processing amss.sin

08/007/2016 15:07:17 - INFO - Checking header

08/007/2016 15:07:17 - INFO - Flashing data

08/007/2016 15:07:27 - INFO - Processing amss_fs_ayame.sin

08/007/2016 15:07:27 - INFO - Checking header

08/007/2016 15:07:27 - INFO - Flashing data

08/007/2016 15:07:27 - INFO - Processing apps_log.sin

08/007/2016 15:07:27 - INFO - Checking header

08/007/2016 15:07:27 - INFO - Flashing data

08/007/2016 15:07:28 - INFO - Processing cache.sin

08/007/2016 15:07:28 - INFO - Checking header

08/007/2016 15:07:28 - INFO - Flashing data

08/007/2016 15:07:29 - INFO - Processing fota0.sin

08/007/2016 15:07:29 - INFO - Checking header

08/007/2016 15:07:29 - INFO - Flashing data

08/007/2016 15:07:31 - INFO - Processing fota1.sin

08/007/2016 15:07:31 - INFO - Checking header

08/007/2016 15:07:31 - INFO - Flashing data

08/007/2016 15:07:32 - INFO - Processing kernel.sin

08/007/2016 15:07:32 - INFO - Checking header

08/007/2016 15:07:32 - INFO - Flashing data

08/007/2016 15:07:35 - INFO - Closing TA partition

08/007/2016 15:07:35 - INFO - Opening TA partition 2

08/007/2016 15:07:35 - INFO - Processing system.sin

08/007/2016 15:07:35 - INFO - Checking header

08/007/2016 15:07:36 - ERROR - Processing of system.sin finished with errors.

08/007/2016 15:07:36 - INFO - Ending flash session

08/007/2016 15:07:36 - ERROR -

08/007/2016 15:07:36 - ERROR - Error flashing. Aborted

08/007/2016 15:07:36 - INFO - Flashing finished.

08/007/2016 15:07:36 - INFO - Please unplug and start your phone

08/007/2016 15:07:36 - INFO - For flashtool, Unknown Sources and Debugging must be checked in phone settings

08/007/2016 15:07:36 - INFO - Device connected in flash mode

08/007/2016 15:07:38 - INFO - Device disconnected

08/007/2016 15:07:39 - INFO - Device connected in flash mode

08/008/2016 15:08:19 - INFO - Device disconnected

08/009/2016 15:09:21 - INFO - Device connected in flash mode

08/009/2016 15:09:42 - INFO - Device disconnected

08/040/2016 15:40:35 - INFO - Device connected in fastboot mode

08/040/2016 15:40:43 - INFO - Now plug your device in Fastboot Mode

08/041/2016 15:41:13 - INFO - Now plug your device in Fastboot Mode

08/041/2016 15:41:29 - INFO - Now plug your device in Fastboot Mode

08/041/2016 15:41:30 - INFO - Please wait device is rebooting into fastboot mode (via Fastboot)

08/041/2016 15:41:30 - INFO - Device will soon reboot back into fastboot mode

08/041/2016 15:41:31 - INFO - Device disconnected

08/050/2016 15:50:46 - INFO - Device connected in flash mode

08/051/2016 15:51:27 - INFO - Device disconnected

08/051/2016 15:51:46 - INFO - Device connected in flash mode

08/052/2016 15:52:11 - INFO - Selected Bundle for SonyEricson Xperia Arc S(LT18i). FW release : 4.1.B.0.587. Customization : Customized MEA

08/052/2016 15:52:11 - INFO - Preparing files for flashing

08/052/2016 15:52:27 - INFO - Device disconnected

08/052/2016 15:52:45 - INFO - Device connected in flash mode

08/052/2016 15:52:55 - INFO - Please connect your device into flashmode.

08/052/2016 15:52:58 - INFO - Opening device for R/W

08/053/2016 15:53:00 - INFO - Reading device information

08/053/2016 15:53:00 - INFO - Phone ready for flashmode operations.

08/053/2016 15:53:00 - INFO - Opening TA partition 2

08/053/2016 15:53:00 - INFO - Current device : LT18i - BX902VRQF2 - 1255-1481_R8D - 1253-8238_4.0.2.A.0.62 - WORLD-i_4.0.2.A.0.62

08/053/2016 15:53:00 - INFO - Closing TA partition

08/053/2016 15:53:00 - INFO - Start Flashing

08/053/2016 15:53:00 - INFO - Processing loader.sin

08/053/2016 15:53:00 - INFO - Checking header

08/053/2016 15:53:00 - INFO - Flashing data

08/053/2016 15:53:02 - INFO - Loader : S1_Loader_Root_773f - Version : R4A069 / Boot version : R9B031 / Bootloader status : UNROOTABLE

08/053/2016 15:53:02 - INFO - Max packet size set to 64K

08/053/2016 15:53:03 - INFO - USB buffer size set to 512K

08/053/2016 15:53:03 - INFO - No flash script found. Using 0.9.18 flash engine

08/053/2016 15:53:03 - INFO - Opening TA partition 2

08/053/2016 15:53:03 - INFO - Writing TA unit 00002774. Value : 01

08/053/2016 15:53:03 - INFO - Closing TA partition

08/053/2016 15:53:03 - INFO - Opening TA partition 2

08/053/2016 15:53:03 - INFO - Closing TA partition

08/053/2016 15:53:03 - INFO - Opening TA partition 2

08/053/2016 15:53:03 - INFO - Processing adsp.sin

08/053/2016 15:53:03 - INFO - Checking header

08/053/2016 15:53:03 - INFO - Flashing data

08/053/2016 15:53:06 - INFO - Processing amss.sin

08/053/2016 15:53:06 - INFO - Checking header

08/053/2016 15:53:06 - INFO - Flashing data

08/053/2016 15:53:16 - INFO - Processing amss_fs_ayame.sin

08/053/2016 15:53:16 - INFO - Checking header

08/053/2016 15:53:16 - INFO - Flashing data

08/053/2016 15:53:16 - INFO - Processing cache.sin

08/053/2016 15:53:16 - INFO - Checking header

08/053/2016 15:53:16 - INFO - Flashing data

08/053/2016 15:53:18 - INFO - Processing fota0.sin

08/053/2016 15:53:18 - INFO - Checking header

08/053/2016 15:53:18 - INFO - Flashing data

08/053/2016 15:53:19 - INFO - Processing fota1.sin

08/053/2016 15:53:19 - INFO - Checking header

08/053/2016 15:53:19 - INFO - Flashing data

08/053/2016 15:53:21 - INFO - Processing kernel.sin

08/053/2016 15:53:21 - INFO - Checking header

08/053/2016 15:53:21 - INFO - Flashing data

08/053/2016 15:53:23 - INFO - Closing TA partition

08/053/2016 15:53:23 - INFO - Opening TA partition 2

08/053/2016 15:53:24 - INFO - Processing system.sin

08/053/2016 15:53:24 - INFO - Checking header

08/053/2016 15:53:24 - ERROR - Processing of system.sin finished with errors.

08/053/2016 15:53:24 - INFO - Ending flash session

08/053/2016 15:53:24 - ERROR -

08/053/2016 15:53:24 - ERROR - Error flashing. Aborted

08/053/2016 15:53:24 - INFO - Flashing finished.

08/053/2016 15:53:24 - INFO - Please unplug and start your phone

08/053/2016 15:53:24 - INFO - For flashtool, Unknown Sources and Debugging must be checked in phone settings

08/053/2016 15:53:25 - INFO - Device connected in flash mode

08/053/2016 15:53:26 - INFO - Device disconnected