i can think of similar situations however it will take you a bit of trial and error to see if any are applicable...
1. not that you are using xrefs or anything but try switching the status of "allow other users to make changes to xrefs".
2. sometimes you cannot save... using the save button. try typing in "save" and you will get a real save not a quick save. this allows you more powe rto push the save back onto the server. it may be worth a try.
3. at times we suspected a clock sync issue that caused autocad or more accurately the computers to tell autocad that the time of access was different than it actually was thus confusing the user access rights. i would check that just out of interest.
4. do some testing with non CAD files. ie try multiple users opening the same word or excel file and see if therre is a conflict detected. if it does not behave as expected then i would speak to the I.T. dept and get them to fix their server.