SharePoint 2007: message "document locked by user or checked out" when document is checked in

Today I experienced the following scenario:

SharePoint 2007 server with a standard document library without versioning and Excel 2007 files. A user could not perform any operations (edit/delete/update) on the document. SharePoint error message appear when trying to edit/delete: the documents cannot be <action> because the document is locked by another user or checked in...

The document library har no versioning activated and the status on the Excel document is NOT checked out, so I could not make the author check it in again. I suspect that a communication error between the authors PC and SharePoint broke the connection between the local copy of the document and SharePoint - thus making SharePoint document status stuck in "locked by another user"...

When opening the Library Settings - Manage Check Out Files the document was not listed there.

Several approaches to this situation:

- checking the local drafts for the user who last edited the document (he used Office 2010 with no local SharePopint drafts folder)
- try a codeplex solution to force check-in and so on

I found an easy way to resolve the deadlock...

Workaround in my case: activate versioning...

1. Open Library Settings for the library with the troublesome document - click on Versioning setting and activate Major Versions - hit OK

2. Now the document can be checked out / edited / deleted again

3. In my case I deleted the file and uploaded the original file from disk

Note: when the author of the document opened the newly uploaded document (with the same file name) he got several options - one of them was to delete a local copy - once this was done he could work with the document in Office as expected.