[OnePlus 3T Guides, News, & Discussion] qbnm:
Thank you very much for the instant answer!

Unfortunately I didn't take notes of the particular files, and I'm not an android developer, but I'll try to sum up what I remember:

Initially it was a rooted phone. I think, I rooted it with these instructions: https://forums.oneplus.com/threads/...sh-twrp-root-nandroid-efs-backup-more.475142/. I'm pretty sure it was running the original OxygenOS 4.1.x firmware. I definitely downloaded it from the official OnePlus site back in 2017 or so. After I didn't update it anymore, because an "over the air" update didn't work anymore (because of root, I think) and I realized I needed to flash it with TWRP. As for TWRP version, I'm unfortunately not sure, if it was 3.0.x or 3.1.x. But up to this point everything worked fine, and the data partition could be mounted without any issues, both on booting into the phone and in TWRP.

Then, 2 weeks ago, eventually I decided to perform the update. I simply downloaded the most recent original firmware version from https://www.oneplus.com/de/support/softwareupgrade/details?code=PM1574156120231, booted into the TWRP recovery and then flashed it over. As far as I know, TWRP flashed the ROM without problems. But booting into it failed with "Encryption Unsuccessful" Error (e.g. https://drfone.wondershare.com/android-issue/android-encryption-unsuccessful.html). First I didn't want to "reset phone", because I thought, it would delete everything on my data-partition, so I first rebooted into TWRP without ticking it. But TWRP was not able to decrypt my partition anymore, it promted for the password instead. Neither my phone PIN nor "default_password" worked.

Then I thought, the old version of TWRP might be not compatible with the recent Android version, so I flashed the most recent TWRP version, it must have been https://eu.dl.twrp.me/oneplus3/twrp-3.3.1-1-oneplus3.img.html. But it also could not decrypt the data partition.

After that I tried a whole bunch of different TWRP versions (incl. some modified), but eventually I found this and some other threads where it says, there's a OnePlus firmware bug in migrating the keys between the keymaster versions or something like that.

As soon as I realized that it's probably not a TWRP issue, I tried to roll back the Android system to OxygenOS 4.1.x, but "Encryption Unsuccessful" error remained.

After that I resigned and simply flashed the most recent Android and TWRP versions again, booted into it and followed the instructions on https://xdaforums.com/showthread.php?t=2450045 to make at least a full copy of the (probably corrupted) partition. Basically I did like

adb pull /dev/sda15 ~/sda15.img

over the USB connection. It took approx. 10 hours or so, but eventually a 121,57 GB big file was created.

After this raw backup finished I booted into an Ubuntu Linux Virtualbox virtual machine and did:

root@db-VirtualBox:~# fdisk -l /home/db/share/downloads/ANDROID_BACKUP/sda15.img
Disk /home/db/share/downloads/ANDROID_BACKUP/sda15.img: 113,2 GiB, 121568210944 bytes, 237437912 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

So I hope, at least the partition itself should be still there. But at this point I'm not sure what and if anything could be done to rescue the data.