[Sony Xperia ZL] tehjero:
The phone belongs to my GF so I'm not aware of any apps that may have been installed that could cause this issue, but she generally sticks with things from the Play store, unless I give her an apk.

I'll try to be as descriptive as I can, but since it's not my phone I may not be entirely aware of some of the more technical aspects since I can't boot it up.

Phone basics

Model: Xperia ZL (C6506)

Bootloader: Locked...AFAIK anyway - phone belonged to my sister who is not tech savvy and gave it used to my GF when her Moto Droid RAZR XT910 (which was my old phone) broke.

Rooted: No (AFAIK, same reason as above)

SIM/carrier unlocked: Yes (I did this at least 3 months ago)

Carrier: Rogers Canada

Plan: PayGo (if that matters)

Here's the story:

A couple of weeks ago she decided to do a factory reset because the phone seemed like it was getting bogged down. After that, her data connection wasn't working, so she contacted Rogers who apparently sent a refresh signal. That didn't work, so she contacted them again last week and they sent another refresh. That still didn't work so she called again this past Monday (Feb 15) and they walked her through resetting the APN; because the phone was unlocked, it didn't automatically pick up Rogers' APN settings, even with her SIM inserted.

However, a mere 2 hours after she was walked through the APN settings, her phone began a boot loop. Every time it boots up it goes through the splash screen then "Android is optimizing apps" then it sometimes gets to the home screen, freezes/hangs, and reboots. It will do the boot sequence running on battery, so I don't think that's the issue.

Bootloader is locked and phone is not rooted and there is no custom recovery (i.e. TWRP) but I was under the distinct impression that Android has a built-in recovery module anyway so I've Googled and tried every possible combination I have known and found to try to boot into it:

Shut off phone, hold Vol- & Power to boot up

Shut off phone, hold Vol- & Vol+ & Power to boot

Shut off phone, hold Vol- & Home button (not hardware, softkey so I hold where it would be) & Power to boot up

Shut off phone, hold Vol- & Vol+ Home button & Power to boot up

Shut off phone, hold Vol+ & Home button & Power to boot up

Turn on phone, wait for pink (or other colour) LED to turn on then hold Vol- (no LED comes on at beginning of boot as far as I can tell)

None of the above methods have given me access to a recovery menu. I've downloaded the Sony Flashtool (Emma) in order to try to flash a stock ROM/firmware but it seems that I can't use it without a registered account and account registration appears to be manually approved by Sony (I'm thinking they limit it to authorized retailers and repair shops?) so that seems to be a dead end.

I downloaded the PC Companion and was able to get it to recognize the phone and perform a repair/update (by holding the flash key, Vol-, and plugging it in while the updater was running, as it instructs to do).

I've done the repair/update twice now. Each time it says it succeeds (update to 10.something, sorry can't recall specifically), then when I boot up the "new phone setup" starts but hangs/freezes and the boot loop starts all over again.

I'm out of ideas...please help!