Raul Prieto writes in with an asymmetrical issue:
All of a sudden when I am trying to save a modified file with a different name in Excel in the same folder as the original, I get the attached reply [see figure] and cannot save it.
I checked in with Raul via email about whether the filename had a / (forward slash) in the name – it did! He was able to change the name in the Finder and save the file. While I wrote about problems saving files in Yosemite, this is unrelated – it’s not a permissions issue, either.
This is a very, very, very long-running problem that seems to crop up only with Microsoft Office apps. Create a file in any other program, put a slash in its name, save it, open it, modify it and save it again. OS X handles it just fine. Do the same with a Word, Excel or PowerPoint document and you receive an error.
Just rename the file and you’re back in business.