I often get a message that an email can not be moved to a folder.

Most likely the filter is broken. Due to other problems with earlier versions of TB102 filters, which are supposed to move messages to certain folders, have lost the destination folder information. You'll have to check your filters, and repair them manually.

After starting up I run the filters manually and every filter works perfectly. The filters trip over something only at TB startup.

Something I'm seeing is that when a filter matches for more than one message being received, it does not run automatically, except for the last message received. The ones received first can then be moved by running the filter manually. See https://bugzilla.mozilla.org/show_bug.cgi?id=1778064

Does this sound like your problem?

Not saying no but not that I've noticed. I'll watch for this. Thanks for the post.

Using TB for a while but recently have two issues with compact folders. 1) I compact folders before closing TB and then at startup TB asks about compacting stating xxx MB will be saved. Why didn't the compact folders I did take care of this?

Your earlier compact must not have had the expected result.

There are reports of such problems. We don't yet know the cause.

2) I use extensive filters to sort emails received. At start up, I often get a message that an email can not be moved to a folder. This seems to kill the whole filter process. From observations I believe compact folders is running because TB always states another process is using the folder. If I wait a few minutes and "Run Filters on Folder" the filters all run properly. It seems to me the filters should recognize the other process using the folder and "wait" for the other process to finish instead of killing the whole filter process.

Automatic compact may be interfering at startup. I would turn it off. Another possibility is AV software.

Let us know the results.