When I try to import messages from Windows Office 365 eM Client starts to do the import and then seems to stop. No crash messages and task master indicate 30%+ cpu activity. The import message (message 111 of 222 [or some such]) reaches the 222 for all accounts but then eM Client starts doing something else - it doesn’t say and I have no idea what it’s doing but a day later, it’s still doing it …
Probably similar to the old https://forum.emclient.com/emclient/topics/outlook-import
You can selectively import data, so maybe try importing just one or two folders and see what happens.