Thunderbird is finding problems when trying to write changes to a mailbox on disk and giving you some possible causes (from a filesystem point of view), that may or may not be the actual cause of the problem.

These problems can be difficult to solve because they could be caused by something unrelated to Thunderbird, such as an antivirus interfering with Thunderbird's ability to make changes to the filesystem. If you have any of that installed, try deactivating it to see whether that makes a difference.