After Effects file saving error

Just clear out All memeory and Disk Cache

Edit -- Purge --- All Memory & Disk Cache...

I tried saving in a new location and saving as a copy but neither worked.

I saw @gangrell04's post below about clearing disk cache and that fixed the issue.

To quickly delete memory and cache:

Go to Edit > Purge > All memory & disk cache.

I usually do this if I get a notice that I'm low on memory space. It's surprising how quickly this fills up. I purged 77GB this time around.

 I tried to change the location of the file of auto-save (preferences), & it works
Hope this gonna help

I could also be that the current HD is full, no space left, so it can't save to its original location, just delete a few things and make space 

Hello. I justa had that same exact issue, and i spent this whole afternoon with a big frustration. But i finally figured it out. I do not have a lot of free space in my HD, so i think that's the reason why.

Just go to preferences and delete de Cache from AE, and i think that is it.

It's also possible that the folder you're trying to save it in needs Admin. permission. 

Save the preset in an other location like Desktop, then manually in the Finder or in Documents Window drag the .ffx file to the Presets Folder.