Logs and screenshots aren’t really applicable here. While loaded into a character I was unable to import lootfilters via clipboard. I was able to import an (out of date) lootfilter from clipboard before loading up a save. The game did recognize XML downloads immediately, although it lags out the UI and hangs it severely for a minute or two (separate issue).
Could you post a copy of the particular lootfilter that you used for this test?
Did you try this with a lootfilter created in game, then exported and edited and reimported or did you use a lootfilter from a third party source like lastepochtools? Or did you use someone elses manually edited lootfilter? Did you use a lootfilter export from an older version of the game?
Again, copies of the file would help us verify the issue by replicating it and obviousy help the devs trace the cause of the problem.
There are instances where corrupted lootfilters (in xml, this can be a single character missing/incorrectly placed) can hang the lootfilter UI in-game. The import function, afaik, doesnt try to correct issues on the fly. The player.log can sometimes show debug messages related to the lootfilter that may explain what could be happening too.