Can't anymore login to MT. It display username and password prompts, but says that all users are wrong - same with telnet/ftp/ssh/winbox. No users were deleted or disabled.

I think it is a password problem, I have had similar problem before and all I did was to reload the OS from a bootable CD and did not keep old config, This was easy because The router OS was running on a PC.
I'll appreciate if anyone out there can give guide to how one can reload the router OS on RB 220 or 230.

I just tried out netinstall for one of my RB200 that has password problem and it worked well. So try this netinstall or installation from bootable CD to reinstal your MT and set a new password. See

http://www.mikrotik.com/docs/ros/2.8/guide/netinstall