I noticed on a new user in userman, that I could login via winbox but not via ssh client (putty). This was a login to the routerboard that was itself the radius server.
When I shorted the password (from around 20 characters to 16 characters), I could then login via both winbox and ssh. It doesn’t appear that special symbols are a problem–the shorted characters I removed were just letters and numbers.
This is for 6.18 on RB1100AHx2.