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First, as you say the problem also occurs on the command line, it is not a TortoiseCVS problem - you will have to go to the CVSNT mailing list.

Secondly, it appears that the problem is directly related to having your sandbox is on a network share - this is not recommended (go to the FAQ section at cvsnt.org, and you will find this: "Do not ask questions about storing sandboxes or repositories on network shares. This is not supported for free. Contact March Hare Software regarding Professional Support options that may be available for you if you require this functionality."