Had a similiar thing happen - locked out of one account on one computer where other accounts worked but that locked account worked on another computer, so it didn't seem like a server problem.
After two days, I thought about the error code led to this in the help file:
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Error 81000314, 81000377 or 81000378: Your contact list is not available
If you receive one of these messages, your contact list is not
available or is in read-only mode. Wait a few minutes, and then try to
sign in again.
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So what worked for me is I went to C:\Documents and Settings\<username>\Contacts\<account> and renamed the contactcoll.cache to contactcoll.cache.bak.
Hope that works for someone else.
