BUG: server says "filename too long" for filename's length of 149 chars!

Why too long??? Linux limit for filesystem is 255 chars… (source)

Latest Server and Client versions installed. Server on Debian 9, Clients on Windows 10.

Yeah, characters, not bytes. Each of your cyrillic characters needs multiple bytes…

Do I understand correctly that a filename limit for Cyrillic (if each char in string is Cyr) is 255/2? Each Cyrillic character takes 2 bytes.