I exported the mbox of a list, in order to test how duplicates are handled. The total size of the mbox (uncompressed) is 20kb. On submitting it from the import tab from settings, the status of the import is shown.
However, the status is still showing as in progress even after some 10's of minutes have passed.
Update: It seems posting a new message on the list, gets rid of the status, so once changed, it goes away.
This issue is still happenning, and from my test, posting a new message on the list did not get rid of the status. This effectively breaks the import feature, since it blocks new imports from happenning, and the only way seems to be delete the mailing list and create it again...
And this issue also happens when nothing is selected before clicking the Import button.
If you email me (email@example.com) with the details, I can fix your specific mailing list manually.