This post is mostly informative to both the adobe community and developers.
Since the release of Illustrator 25.1 I've been unable to save in .ai and/or .eps on my local disk (HDD/SDD), so I've been forced to save everything in Adobve ccloud (.aic).
However, I recently seemed to find the issue: when choosing .ai/.eps as saving formats, I am forced to unlock and make ALL layers visible in order to be able to save on ANY local disk. Otherwise, it will throw the famous "An unknown error has occurred".
Some files I have been working on and saving for days without problem and suddenly the "an unknown error message has ocurred" pops up (maybe 10 minutes after successfully saving the file) and from there on I'm unable to save/save as the file. I have to take everything to Photoshop, paste as intelligent object, close & open illustrator, open the intelligent object, take it to a new file and then save it. This is a real pain and unacceptable, as it renders the working process unefficient and makes me lose work constantly. Sometimes the same problem file later on will open and save without issues again, sometimes the error persists, it's a real lottery.
Mac Os 10.15.7
No hidden/blocked layers or objects (sometimes it's the simplest vector file with only 1 layer)
Ocurrs with local disks and external hard drives
I really hope somebody at Adobe takes this issue seriously - as I can see looking at the forum there are posts from as far back as 2014 without viable solutions given.