I have just done a test. Im using Illustrator 22.0.1 on an iMac 10.13.2 and have the same warning message. I thought this was a conflict with DropBox, but even after quitting DropBox, I still get the warning. Its a software conflict problem somewhere. I think it needs someone smart than me to come up with the answer.

i also get the same error every time while i trying to save my file during progress. It happened once I update High sierra in my iMac.

Adobe Illustrator CS6 version 16.0.0

macOS High Sierra
Ver. 10.13.2

I'm getting the same error and I am saving to the computer

I get the same error message when saving an AI file:
"This file has been modified outside Illustrator. Do you want to continue?"

I'm not saving to Dropbox or any other network device.

Illustrator version 22.0.1
macOS 10.13.1
13" MacBook Pro model identifier 8,1

I am having this same issue and I am not saving this file to dropbox. However my work-in-progress files are in a folder that is sync'd to my Google Drive (Google Backup). I am running the latest version of AI and running on High Sierra. I also use the latest version of Suitcase for font management.

I'm working on a large project in Adobe Illustrator and am worried that something is going to happen to the file I'm working on.

Same here. Just updated to High Sierra 10.13 2 days ago and since then every time I save an Illustrator file, I receive the same message. I do have the files saved to iCloud Drive, not Dropbox, but I am the only one who using the files.

Just started happening today. I'm on an iMac running High Sierra, saving to my local drive. After saving and naming the file, I get "this file has been modified outside illustrator. do you want to continue?" about ever other save.
No idea why.

So, after all this time.... no solution, fix?

While I can't commit to a call, I can tell you that I'm getting this error message working off local files on my laptop, not on Dropbox or Google Drive.

After upgrading to the latest Mac OS I now get this almost everytime I save an Illustrator file to my internal hard drive. Please fix this :-(

Me too. Not saving to Dropbox, although I do have Dropbox. I am opening and saving to the hard drive. But this happens now with Illustrator almost every time. Not all the time, but when it does, it seems to take a looooonnnng time to save. High Sierra, 10.13.1, 32 GB Ram, 4 GHz Intel Core i7.

I updated to High Sierra yesterday and am now getting this error. I'm saving the files in an ordinary folder on the machine, it is not in a Dropbox folder or any sort of cloud drive.

I've got that problem too

So glad to find i am not the only one! I am having lots of these same problems and it's been driving me crazy!

HAVING THE SAME PROBLEM. I have a master Ai file saved on my HD. I made a new one in Illustrator 2018.
Copied my art from the old file to the new file. So far it seems to be working. Haven't seen that pop-up saying file was modified outside illustrator. Whenever I start a new job I always go to that master file. I have the palettes for of all the symbol, brushes colors etc., cleared. Hope this helps.

I have this problem when working with files in synced Creative Cloud folder (not using Dropbox or any other sharing service). Problem goes away when I quit Creative Cloud app.

Same issue with me as i am saving to dropbox but not when i am saving to desktop. Also after upgrading to HS and the latest AI

Same here ever since I upgraded. This is very annoying. I also save to DropBox but everything was fine until I upgraded my Mac software :/

Getting this message when saving to desktop or network server. Work around has been "Save As" and overwriting the file every time you want to save a file.

I get this error almost every time I save it now. Could this be due to the new APFS files system in High Sierra?