'Cannot connect to server'

Dear all

Suddenly (well, I am unsure how long it has been going on, and I have made some changes to my computer) my Linux machine (Mint 18 x64 Cinnamon) cannot sync files with my Windows 8 machine. The latter runs my Seafile server. The former reports - when I hover about the link icon in the main Seafile client window - ‘cannot connect to server’. This happens even when the two machines can see each other in other ways (I have home network links between the two) and even when the respectively firewalls on each computer are disabled; and I am not currently running any other real-time security software - and, previously, everything worked.

Could someone help me troubleshoot, please? Thanks.