C:\Users\[name]\AppData\Local\Temp\mft2645.tmp\icons.tmp was not found

[Zaya Moone]

I was working on a project and everything was acting normally, until one of the times when I tried to run the frame and I got this error. I tried again and the same thing happened, so I thought that closing Clickteam might fix it. However, I tried to save my project before closing and I got the error again. I haven't saved the project since I started it (I know, I'm an idiot) and I really do not want to lose my progress. I have no clue how to fix this issue; I've found other threads on it but they were either unanswered or were too confusing to understand. Anybody able to explain how to fix this?

[CT_Yves]

You can't fix this. This means that an important temporary file used by Fusion for your application has been deleted in the background. Your changes are lost, I'm sorry... Have you cleaned your hard disk while Fusion was open? Or do you have an automatic cleaning app running in the background?

[Zaya Moone]

I probably do have one, but I don't know what it would be called...
And no, I did not clean it. Do you know of any apps that would do this? If so, I can see if I have any of them
If I did, what would I do to prevent it from happening again?

[Wodjanoi]

Antivirus software/real-time scans for potential threats or any process that can forcefully delete files whenever there is a chance(usually all antivirus tools have multiple options to make scans less agressive or allow/whitelist specific locations and files).
It seems that Fusion 2.5 .tmp files are vulnerable/unlocked for a brief moment when you run the Frame or Application.
For testing purposes I forcefully recreated your problem scenario with a little .bat script:

cd "%HOMEPATH%\AppData\Local\Temp"
:DeleteTmpFilesForcefully
if /i exist icons.tmp icons#aa.tmp /s (
del /s icons.tmp icons#aa.tmp 2>NUL
goto :DeleteTmpFilesForcefully
) else (
echo icons.tmp deleted )

Run the .bat, a mfa project and also watch icon.tmp in the Temp subfolder, then run your Application/Frame a few times and icons.tmp will die very soon - sayōnara save functionality. :)