I had the same problem with one user in my BES. I did the /cleanreminders and then deleted and recreated the user's mail profile and that did the trick.
are you using roaming profiles? those are a major source of profile corruption. glad you got it figured out and once again, the BB continues to work flawlessly while M$ products take a dump.
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~Di~
Windows 2003
Exchange 2003
BES 4.1