I am reporting the same issue for v.3.30 as reported in v.3.29 with regards to the error message "Unable to create intermediate backup".
The only difference is, the details regarding the different options of saving the file and the full path did not correct the situation.
In this case, the file exists on a server, Windows Server 2008 R2 standard Service Pack 1.
I have double checked security, full rights on the source and target directories are in place.
It must be something where the file was not closed properly from the previous use.
A solution that I found to work, copy the file: i.e. copy a.psafe2 a2.psafe2 Opening the second version of the file allows editing without any other changes.