Hello. I am experiencing the same (?similar) problem. In my case, the problem is restricted to MS Project 2010 files received as an email attachment. I receive the file and open it directly from Outlook 2010 (i.e. I don't save the attachment locally before opening it). I can then edit the MSP file. I cannot 'save' the file in MSP as the file is opened from Outlook as read only. So when I try to 'save as', I get an error message [The file "xxx.mpp" cannot be opened. Check that the file name and path are correct..."]
The file name referenced in the error message is the new file name I am trying to 'save as', not the original file name I originally opened from Outlook.
The file size is only small - less than 1 mb. Also occurs with a file of 316 kb. So I don't think file size is the issue.
I have check my MSP options as described in this thread:
- Options > Save > Auto save every = unticked.
- Options > Display > Entry Bar = unticked.
- Options > Advanced > Edit directly in cell = ticked.
Can anyone advise if there is a fix for this problem? Is it covered in a service pack?

Thanks