I’ve just tried to download a whole folder as zip over the webinterface.
The downloaded zip was exactly 0 bytes big, which is a bit small ;-).
In the server logs I’ve found following:
2020-02-25 17:44:07: ERROR: Error while adding file “/var/lib/urbackup/example.com/200225-1507/mail/ich/mail/cur/1349275128.M615396P20421.example.com,S=2402:2,S” to ZIP file. Error: invalid filename. OS error: Code 0
2020-02-25 17:44:07: ERROR: Error while adding files and folders to ZIP archive