Hm, seems like this was related to a local corrupt file as the inbox also stopped working, and bottom-left corner log readout pointed to error-log, which in turnlists:
20/05/2017 12:14:03 - File open failed: 'D:\folder\folder\file.flac' - D:\folder\folder\file.flac,3
20/05/2017 12:14:03 - refresh library failed: D:\folder\folder\file.flac
Once I removed the corrupted file MB inbox started to work again, and I would guess missing files warning would have popped-up again as well (but I have corrected all of them by now though as it was an intentional move/rename...).
Cheers.
c.