Permission errors, I'd guess. Where did you install mIRC to when running it from the non admin account? Is your mirc.ini located in program files\?
Type //echo -a $mircdir in mIRC and paste the results here.
I get:
C:\Users\<myname>\AppData\Roaming\mIRC\
Although I can run mIRC from C:\Program Files\mIRC
ETA: On mIRC v6.31 and v6.3, I get Connection Refused rather than Unknown Error.
