4.19.42+ kernel broke the file system, LZ4 issue, how to revert to prev kernel?

[Gyuunduh]

I'm sorry for not being able to access the log files for exact commands or error messages i've ran and had before running into problems. Also I feel I'm a bit out of me depth in here, having learnt to tinker with Linux a bit with Xbian but having no proper understanding of cmdline.txt or boot blocks, I only know how to use btrfs-auto-snapshots. Anyway here goes:

I have been running an Xbian installation at least one year old, not paying any attention to filesystem compressions or messing with them as everything has been working just fine. The latest kernel update on my Raspberry Pi 2 (4.19.42+ I guess) didn't work, apt-get upgrade gave me an error message saying: "you have LZ4 compression enabled, but this is not longer supported by kernel v4.19.42+"

The advice apt-get gave me was to run apt-get --remove-compression or apt-get --recompress or something like that that had compression in it. I dont' recall that exactly and I can't check what it was because the log files are out of my reach now.

I ran the command, and an ASCII visualization of the process showed up. It failed at 75 %, error message saying something about there not being enough space, running out of kilobytes maybe (that sounded legit, I had something like 4 GB of free space on 100+ GB SD card).

I tried running apt-get upgrade again anyway after that and this time it proceeded to unpack and install the newer kernel. But after reboot Xbian boots into recovery console, failing to mount root, telling me:
"wrong fs type, bad option, bad superblock on /dev/mmcblk0p2, missing codepage or helper program or other error." And later: "The root partition as specified in cmdline.txt or boot.src.txt is now mounted under /rootsfs", and that the directory is totally empty.

I'm pretty sure it has something to do with the kernel being upgraded to a version that doesn't support LZ4 compression but the file system still being compressed and thus being unaccessible.

Is there any way to revert to an older kernel that still supports the compression, so I can access my Xbian filesystem? Or is the problem somewhere else?

Anyway, I'd be really grateful for help, because right now it seems that I can't access any files on my Xbian.

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