Also up to date - still happening.

I am up to date and it is still happening every time I save a file.

I had the exact same issue; I've always not used iCloud or DropBox on this particular Mac (10.13.6). I unchecked HandOff and made sure no files are linked to iCloud as Claudio Escobar but the pop-up persist. I then remove ICloud Device from Finder's Favorite then the pop-up goes away - not entirely sure why perhaps that has to do with the file system in 10.13.6.

I have the exact same issue as what Claudio Escobar mentioned below, must be a problem between Illustrator and iCloud backing up.

Problem persists on the latest build. Very annoying. Interesting observation from Claudio Escobar (June 9) below, maybe you should read it Abode?

The people at Adobe have known about this for years, and hey they just dont give a ****. I have been clicking thru that warning for years

I'm using CS6 with High Sierra and Dropbox. All files result in this alert when saving. How is updating the software a fix??? Please tell us why it is happening.

"Completed"
The whole point of this subscription model is that you'll be able to fix things more efficiently. Hard fail.

Im Running Mac OS 10.13.5 and Illustrator 21.1.0 - This problems seems to happen when I save the file to an iCloud folder on my hard drive, as soon as I re saved the same file to another folder that is not related to iCloud the problem went away.

I have the latest version, updated recently again also to ensure this, and it keeps coming up

I have the newest version of Illustrator, and I'm still getting this error message when saving to my network server.

I am getting this message while running 10.11.6, El Capitan OS.

I have updated to latest version and still getting this message.

The BETA does not resolve the issue of the message "This file has been modified outside of Illustrator". I just installed it and the problem showed up immediately. This is truly annoying. If it is related to the latest Mac OS update, then it would be good to know that for sure. So far, Apple seems to think it is an Adobe problem. It only shows up in Illustrator, at least in my experience so far.

The fix works for me, thx heaps it was driving me nuts!

I don't think this is an Adobe problem; I get the message saving a file in TextWrangler to my HD even immediately after creating a new file. It must be some problem with the Mac OS 10.13.3 update...

Same problem.. saving to HD. Driving me crazy....

I'm also having this issue and am not working on a network or saving it to the cloud, online drives, or a removable drive. I'm just saving it to my machine's hard drive. I would say it happens 90% of the time when I use Illustrator.

I'm having the same issue.
Not saving to Dropbox, just on to my hard drive.
Doesn't happen on every save of the same file. Seems pretty random.

Same issue here, I'm just saving to a drive. Every time I copy and paste (and yes I embed the file) items I get this error. It drives me nuts.