I just faced and fixed exatly the same problem on my Galaxy S3 I9300 with Android 6 Marshmellow (bliss 6.1 rom). Bootloop happend without any warning at the Night of 31st to 1st November. Just saw my phone rebooting ever and ever. Over the last 5 days I tried to get it back working or even to get some more backups out of the twrp backup.
What I found was: I got it running with factory reset and right after installing either of them: telegram, davdroid, the phone crashed directly in the appstore while installig. Then the phone rebooted and got stuck.
Important: you should make a twrp/nandroid backup if you not already did.
a lot of reading, testing and questions in xda got me that far and this link here finally brought me to the ida: TWRP backup - Can I restore some app data without restoring the whole image?
extract all neccessary /data/data and data/app subfolders of your apps in the system and restore them to a clean factory reset.
This worked not perfect but I got it running to access some more data as before. I did the copy or delete of the app folders through twrp -> advanced -> file manager.
After I later got another(quite simple) idea on the base of initial guest post: why not going the way in the other direction and removing apps in the broken installation.
This did the trick perfectly. I finally got into my broken rom again, all remaing apps worked as before (quite important to me since encrypted apps like threema and signal are not just copy-data-backups).
Right now I am doing backups of my old Android 6 system :) for the import in my new Android 7 setup.
Hopefully it works for you, too!
OK now step by step what I did:
- be sure to have a twrp backup of your device (efs,boot,system and data) and to have your broken system installed and not factory reseted.
- then in twrp got to advanced -> filemanager and navigate to systemfolder. there delete all unneccessary apps by deleting the appropiate folders in /data/app and in data/data. This will "uninstall" the apps. These will not be optimized or loaded when you boot up. I especially deleted Telegram, DavDroid since I knew out of my checks one of them was causing the error. but i just deleted more apps that I do not need a backup or which are less important. For me this did the trick! Even better this can be done 1 after 1 if neccessary. I first tried this with all apps -> again bootloop. Tried it again with less apps -> worked. So its likely not the system its directly a app in the optimizing/starting process killing the bootprocess.
- just to be sure clear cache and dalvik cache to be sure android is reloading the changes and not seeing and cache
- then go for a reboot to system (Fingers crossed)
- if your're able to boot in again, then imediately open google play store -> scroll down to settings and deactivate automatic update of apps ( Just to be sure not getting any update causing a crash while you are inspecting your data, making updates and so on)
- finally i would recomend thinkig about moving on to a Nougat rom (if available for you). It's likely that there will not be an update soon fixing this bootloop problem (even if its unclear what caused it or which app(s)). I choose ressurection mix since its a quite active project and frequently updated.
best luck to you guy, I know how hard this problem hits.Alex.
(Guest "Alex." and me are the same author -just forgot that I already have an account).