The bug
Trying to back up a world with the games built in backup function will fail if the datapack folder of the world in question contains a symlink to a datapack that is stored in a different location.
Possibly related to MC-136870.
Linked issues
relates to 2
Comments 2
This seems to be partially fixed in 1.20 Pre-Release 7 (or earlier, I haven't tested for a while). If the datapack the symlink points to is a .zip file the backup works now. If the target of the symlink is a directory it fails to back up the datapacks, but at least it backs up the rest of the world now.
Happens on 1.16 and Windows too with proper NTFS symlinks, not junctions. Additionally, the behaviour that it just proceeds with converting the world even though the backup failed is unacceptable.