I recently switched from Pop!_OS to Kubuntu, and my world created in Pop!_OS is getting the "Locked by another running instance of Minecraft" despite there being no other instances running.

Creating a new world works as intended.

Deleting "session.lock" in the world folder makes it possible to enter the world, but the world completely resets (but I do keep my inventory).

World link, 'Geralt' is the latest, and the others are backups. - https://drive.google.com/drive/folders/1Wfg_fNwYhhDL84QpAGfTd46266xkv5O9?usp=sharing

Edit: I had a friend download the world. They created a backup from the "Edit" screen and sent the backup to me, and I was able to get back into the world.