Can you show a screenshot or the error message or an error log perhaps?
I did a search and saw that this topic has been posted a few times here, this was the last answer I could see from a SketchUp team member:
‘This issue has been linked directly to Livedrive and the software component they have that affects anything saved to the L: drive. The only fix is… don’t save directly to the L: drive. I recommend this for all cloud software at this point, just to be safe… save to your Desktop or a local location on your C: drive and only copy it to your network or cloud share service at the end of the day as a backup. (To be clear, I’ve only seen this happen with Livedrive… but it’s better safe than sorry.)’
Although in this answer it references Livedrive, others seemed to be having problems with Dropbox too.
Edit: Sorry I only found an answer for CfileException 2 and CfileException 3, not CfileException 13 as you had asked, my bad!!
Edit 2: It seems that there was one post on CfileException 13 and the author points to the thread on CfileExtension 2 relating to trying to save to a network drive rather than locally. So my first answer may actually turn out to be correct? SketchUp is not Letting me save Are you trying to save to a network drive or cloud storage folder?