It is a deep area where your IT staff gets to work. I have yet to see a single cause or cure but most often it's some permission issue or the "server" has run out of handles. There are also patches and hotfixes that MSFT only shares with you if you call in.
As you have done the usual it's time for the IT staff to call it in and gain access to the stuff MSFT doesn't put on the web.
I have a user who accesses her excel docs thru a mapped network drive. Sometimes (and it doesn't happen all the time) when she goes to save the document she gets the following error:
"Someone else is working in <drive:\path\document_name.xlsx> right now. Please try again later."
1. There is no one else working in the document. It has happened with docs she has created herself.
2. She has all appropriate permissions to the share. I have even added her as a domain admin to see if this alleviates any permission issue, but it still happens.
3. Her docs were originally on a Server 2008 box. I moved them to a Server 2003 box and it problem stopped for about 3 weeks. Now it's happening again.
4. She can save to the local PC with no problems.
She's using Windows 7 x64 and Office 2013 x64. It also happened using Office 2007.