After upgrading to High Sierra, I'm getting this error message about 30-50 times a day:
Is there any way to find out what is causing this error? Any ideas how to fix it?
So none of the given answers works on High Sierra (OS X 10.16.x) and I don't have Clean My Mac 3. This Apple article did the trick for me and pretty easily: https://support.apple.com/en-us/HT203538.
This involves setting the permissions through the finder and then resetting the permissions by using the command
diskutil resetUserPermissions / `id -u`
Make sure you cd to the appropriate home folder (I did this logged in as myself for another user that was experiencing the issue) to avoid access errors.
If this step doesn't work, try the second
You don't need both steps, if one works, you're done.
It appears that the problem lies with Permissions, yet High Sierra (10.13.3) has removed access to 'Repair Permissions' in Disk Utility.
A neat way round this is to use CleanMyMac3. In addition to its basic role of cleaning unwanted files from your Mac, it has an option of Maintenance which offers 'Repair Permissions'. This took just a few seconds on my 13" MBP and the problem is solved – no hassle, no use of Terminal. I'm mighty impressed with this program.
Faced the same problem for many days. After upgrading to Mojave.
Disk utility repair was only a temporary fix.
Then finally found this link. It did the trick. https://support.apple.com/en-us/HT203538 Essentially, resetting user permissions.