Mac & Windows clients stop sync

I have VERY annoying problem with one user account - through web browser everything works ok, but with desktop clients, both windows and mac sierra, they refuse to sync. Directory lists are received ok by client, but when it starts to download both clients give out error messages and refuse to continue - it’s like there is something wrong with filenames.

Just to make things bit more fun, this problem is only with one user, all others work a-ok with Win10 and latest client software.

I’ve noticed that’s quite common problem, but I would very much like to find solution.