File is locked for editing by ' another user '

With some regularity a user  opens a file that is locked for editing by ' another user '. Normally you expect here, instead of "another user", a user name (eg. Peter), especially if this is correct filled out in the personal settings of Excel.

It also happens that a user (John) receives the message that the file is protected against editing by "Peter", while Adrian is editing the file?!

Does anyone have an idea how we can fix this? We work with MS office Pro Plus 2007 on Windows server 2003.

Thanks in advance.

Adri.