We need more details to cipher this out.
To WHOM where you sending a PM? They may have such options turned off. If you don't want their name out there, you can PM me.
WHAT were you sending in a PM? There are some keywords that bugger the software so it freaks out and gives that message. Have you tried to send the same message to anyone else and have it go thru or give the same error? Wanna try sending it to me? You may even want to do some trial and error, sending bits at a time, to see if there is a keyword throwing the system into a tizzy.