I just experienced this issue, I had a user that i could not activate. It kept coming up with the a generic “An error occured, please contact your system administrator” error. I ensured that all the proper security persmissions for the BES admin account were set. I was able to activate other users fine using the same blackberry. No errors in the event log. Rebooting the BES and Exchange server didn’t help.
Here is what I did to resolve this:
1) Remove the exchange attributes from the account and then delete the account
2) Create a new account without an exchange mailbox
3) In Exchange system manager run the mailbox cleanup agent which marks the old mailbox as orphaned
4) Re-connect the exchange mailbox to the new account
Voila, activated like a charm