My Addin says file can't not available

[Pat_The_Bat]

.... is open in another application. Please close it and try again.

and

Microsoft Excel cannot access the file "filename".
There are several possible reasons:
>The file name or path does not exist.
>The file is being used by another program.
>The workbook you are trying to save has the same name as a currently open workbook

Then I get a red circle with a white X that simply says "400" for the error code.

Any direction is appreciated.

 

[sijpie]

Which addin is this? Do you know what file it is looking for?