How to analyse this error: "Database open error: D:\Lotus\domino\data\mail\xxxx.nsf Database is currently in use by you or another user"

Hi all, I'm often receiving this error message on the domino console. I have tried following things: *Disable backup *Disable diskeeper (defragmentation tool) *Disable antivirus We are running Win 2003 R2 and Lotus Domino 8.5 HF460 Does anyone have some advise how to resolve this issue? greetz, Dave