[Xperia SP Q&A, Help & Troubleshooting] Sheepy27:
Ok it was the Volume DOWN button, it worked to get into flashmode BUT the flashing was aborted! Why is this so complicated?!

Here the log files from FlashTool:

28/009/2014 22:09:21 - INFO - Device disconnected

28/011/2014 22:11:58 - INFO - Selected Bundle for XPERIA SP-C5303. FW release : 12.0.A.2.254_IR. Customization : GENERIC-BABAK

28/011/2014 22:11:58 - INFO - Preparing files for flashing

28/012/2014 22:12:24 - INFO - Please connect your device into flashmode.

28/012/2014 22:12:33 - INFO - Device connected in flash mode

28/012/2014 22:12:34 - INFO - Opening device for R/W

28/012/2014 22:12:34 - INFO - Reading device information

28/012/2014 22:12:35 - INFO - Phone ready for flashmode operations.

28/012/2014 22:12:35 - INFO - Current device : C5303 - YT910JKF86 - 1278-6195_R5B - 1269-3538_12.1.A.1.201 - TELEKOM-LTE_12.1.A.1.201

28/012/2014 22:12:35 - INFO - Start Flashing

28/012/2014 22:12:35 - INFO - Processing loader.sin

28/012/2014 22:12:35 - INFO - Checking header

28/012/2014 22:12:35 - INFO - Flashing data

28/012/2014 22:12:35 - INFO - Processing of loader.sin finished.

28/012/2014 22:12:40 - INFO - Loader : S1_Root_7054 - Version : R5G006 / Boot version : R11B023 / Bootloader status : NOT_ROOTABLE

28/012/2014 22:12:40 - INFO - Processing partition-image.sin

28/012/2014 22:12:40 - INFO - Checking header

28/012/2014 22:12:40 - INFO - Flashing data

28/012/2014 22:12:43 - INFO - Processing of partition-image.sin finished.

28/012/2014 22:12:43 - INFO - No bootdelivery to send

28/012/2014 22:12:43 - INFO - Processing kernel.sin

28/012/2014 22:12:43 - INFO - Checking header

28/012/2014 22:12:43 - INFO - Flashing data

28/012/2014 22:12:49 - INFO - Processing of kernel.sin finished.

28/012/2014 22:12:49 - INFO - Processing fotakernel.sin

28/012/2014 22:12:49 - INFO - Checking header

28/012/2014 22:12:49 - INFO - Flashing data

28/012/2014 22:12:52 - INFO - Processing of fotakernel.sin finished.

28/012/2014 22:12:52 - INFO - Processing amss_fs_2.sin

28/012/2014 22:12:52 - INFO - Checking header

28/012/2014 22:12:52 - INFO - Flashing data

28/012/2014 22:12:54 - INFO - Processing of amss_fs_2.sin finished.

28/012/2014 22:12:54 - INFO - Processing amss_fs_1.sin

28/012/2014 22:12:54 - INFO - Checking header

28/012/2014 22:12:54 - INFO - Flashing data

28/012/2014 22:12:55 - INFO - Processing of amss_fs_1.sin finished.

28/012/2014 22:12:55 - INFO - Processing amss_fsg.sin

28/012/2014 22:12:55 - INFO - Checking header

28/012/2014 22:12:55 - INFO - Flashing data

28/012/2014 22:12:57 - INFO - Processing of amss_fsg.sin finished.

28/012/2014 22:12:57 - INFO - Processing apps_log.sin

28/012/2014 22:12:57 - INFO - Checking header

28/012/2014 22:12:57 - INFO - Flashing data

28/012/2014 22:12:58 - INFO - Processing of apps_log.sin finished.

28/012/2014 22:12:58 - INFO - Processing cache.sin

28/012/2014 22:12:58 - INFO - Checking header

28/012/2014 22:12:58 - INFO - Flashing data

28/013/2014 22:13:03 - INFO - Processing of cache.sin finished.

28/013/2014 22:13:03 - INFO - Processing system.sin

28/013/2014 22:13:03 - INFO - Checking header

28/013/2014 22:13:03 - INFO - Flashing data

28/014/2014 22:14:11 - ERROR - Processing of system.sin finished with errors.

28/014/2014 22:14:11 - INFO - Ending flash session

28/014/2014 22:14:11 - ERROR - ERR_SEVERITY="MAJOR";ERR_CODE="0009";ERR_DYNAMIC="Block verification failed 0x300005c8";