runtime error 1004 method save of object _workbook failed

I have a question. I am taking over a project and once and a while I get the following error message:
runtime error 1004 method save of object _workbook failed
I was wondering if this bit of code might be the problem. This workbook is shared by multiple users and is protected. I don't get the error all the time, so i'm a little confused.
I can't post the workbook because of confidential info. here is the save code. any suggestions would be helpful.

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In B15 is the shift and date which they enter from B3 and B5. B15 is what the file is save as. EX:First_shift 6-13-13

I forgot to mention when the user get's this message they can't select the file tab. They are not able to go backstage.