when trying to save the config I get:
"the os version of the firebox or configuration file does not support multiple radius servers"
this is the first and only radius server
policymanager 12.5.2 / OS 12.1.3 / XTM330
ok , just found out, that the radius server must be named RADIUS not radius or other name
That appears to be a restriction of Fireware 12.1.3 and older. For newer models and/or firmware, it is not true. I have multiple 12.5.2 Update 1 T35 models with no authentication server at all, and I set them up with "AuthPoint" as the RADIUS server name. My own T35 running 12.5.2 U1 has only "AuthPoint" and "DuoSecurity" RADIUS server names.