Sharepoint 2010 workflow error, unable to open file

I created my list and workflow that starts automatically on new entry on list. I have another workflow that checks my list and waits for my column to come be a specific status. Then that workflow create entry in doc library.

After creation I tested it a lot and everything was great.

But in production I get errors randomly. Approximatly 50 users use that list to create items.

In the history list I get the error:

Workflow could not create the item list. Make sure the list exists and that the user has permission to add items to the list.

In the log files I found errors that say:

unable to open document ("name_of_document.docx") 

and some error number:

System.Runtime.InteropServices.COMException (0x80070050)

It's not a permission problem because these error appear rarely -- most of the operations ends successfully.

Appreciate for help.

the problem could be that two process are trying to access for create the document and both create the same filename. Try to modify the name of generated file adding more data to the name in order to differentiate the file. Dig into the server for more information about the exception