This is an old problem but it could now become a show stopper if not solved.
When run as an administrator PS runs without a hitch on my system. But I shouldn't have to do that for obvious reasons. If I run it normally (not as an admin, even under an admin account), it misbehaves and strange things are happening :
#1 When specifying scratch disks, only C: is listed. All other disks/partitions are ignored (not listed). No problem when running as an admin. Of course, these disks/paritions are recognized by any other app running at the same time under Windows. All these disks and partitions have a lot of free space available. I'm very curious about how PS decides what disk can be listed as a possible scratch disk.
Moreover, PS is also unable to create and write to the system's and user's TEMP folders (although full rights have been given to all users - other apps can access these folders without any problem). This problem could explain issue #2.
#2 It seems unable to read or write its preference file. Any preference changed in the preferences dialog is not retained when exiting. No problem when running as an admin. Needless to mention that all necessary permissions have been checked.
#3 It is unable to write any edited image file to any other disk than C:. No problem when running as an admin. Any other app can read from and write to these disks.
#4 When exiting PS, the photoshop.exe remains active forever after the main window has been closed. It is waiting something from the network, permanently sending TCP/IP requests. When run as an administrator, photoshop.exe terminates within a few seconds after the main PS window has been closed.
I went through all the recommendations made on this page : https://helpx.adobe.com/photoshop/kb/basic-troubleshooting.html . Really. To no avail. However, it's worth noting that if I launch PS while pressing the famous Ctrl+Alt+Shift key combo and answer NO to the question about deleting the preference file, everything is working fine. But if I relaunch PS normally, the problems are back.
I have also created a new standard account, a new admin account and tried to run PS under the system admin account. No difference.
Additionally, I also checked the Windows Defender settings, just in case.
Since there's no problem when running PS as administrator, it's obvioulsy a permission problem. No other application on my system needs to be run as an administrator. I have checked all the permissions that have been listed in various articles coming from Adobe. So, obviously again, these documents are not accurate enough. Something is missing in the list of system objects to which full access must be given.
I should also mention that I have a similar problem with Bridge and Lightroom, but let's focus on PS for now.
Please refrain from telling me to re-install everything Adobe (already done) or to point to a possible system misconfiguration. My system is well maintained and works fine with the many applications installed on it and as many of us already know, I'm not alone with these kind of problems lasting since years.