Have you had another user log into his machine and attempt to reproduce the error? Preferably a user with the same permissions as he has. It would be helpful to post some environmental information(OS of the server?), as well as a snipit of a ULS or Event log of the error. By the little reading that ive done, it seems this error can present for multiple reasons. Do you have any custom code or web parts integrated into the site? Have you done any recent updates to the server, and has this specific users permissions changed lately?

We are running WSS 3.0 SP2. We have 1 single user that started having the following occur: When he goes into a document library in which he has contribute permissions, selects a file, picks Check Out and Edit, he is getting a pop-up stating 'Object reference not set to an instance of an object". When he clicks on the OK button he gets another message stating that it may be a permission issue or the file may be locked for editing by another user. I have confirmed that nobody else has the file checked out for editing. Other users (including myself) can open the exact same files, check them out and edit them, then check them back in. I have searched the web for this error and have not found anything useful to tell me what to try on his machine. I’m thinking it is a local PC issue since he is the only one experiencing the issue plus I have had him log into our Citrix client and he can successfully check out and edit files there. He has done a reboot of his machine but this did not resolve the issue. Looking for any help I can get!