Sausuke
While I don't know the exact way to solve this, I suggest you do the workaround:
1. Using PotPlayer portable, put it in different drive (e.g. in drive D or other).
2. Go to that PotPlayer portable and make the shortcut for it (right-click the .exe and choose "create shortcut", you can rename the shortcut to whatever you like).
3. Cut and paste that shortcut to your Start Menu (C:\Users\<your username>\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\, change that <your username> with the actual your username folder, make sure to configure "show hidden folder" because AppData is hidden.).
4. The shortcut should be appear in Start Menu, now open the PotPlayer portable itself and press F5 to open the configuration.
5. Look for Association tab over there, and choose your preferred video file extension to played with your PotPlayer.
The above are just workarounds, to really solve the problem, you need several different player to check whether this is only limited to your PotPlayerMini.exe, and try to play with and without the SVP and observe what it gives. (You can try to use MPC-HC, MPC-BE, VLC etc, all of those don't have to be installed).
If you can remember, maybe you want to trace back when the last time you can play it fine and check whether at time you installed something or do something with control panel.
I never have problem like this so I don't know for sure what causing it