@Cass
I`ve tried another client, as someone told me that a newer version of RealVNC client might be incompatible with the old Amiga VNC server.
Nope. If there's incompatibility it's the server side. RFB protocol is very well defined, and future extensions don't break things. Naturally it's possible that Avnc implementation is broken.
I`ve downloaded and installed the TightVNC (a new version also, but it might handle the connection/trasmission of data in a different way), but no luck.
I prefer TightVNC since it does the most extensions (compression types etc). However, if the server doesn't support these extensions it doesn't really matter if you use RealVNC or TightVNC. With basic connectivity they work the same (in fact they're based on the same source tree originally).