After having made more tests on the INSERT INTO SQL, I think I have found the reason I got the "File is in used by another user.".

When I mentioned that, for those failed that occured twice in a month, the length of the transaction was 17 and 18 seconds, I think it was related to the time the PHDBase library is taken to update the PHDBase tag. PHDBase is locking the table and this might explain. While doing some tests an hour ago, the same situation occured when 2 users tried to create a message. When I close my Visual FoxPro application, they were able to add again.

I will check that with Jim Korenthal.

Thanks for all your help.