There’s no secondary logfiles on the server. My first advise is to check if you have upload filters and disable them to see if the problem is with one of those.
If that doesn’t help, do you have an example picture that fails for you? Maybe I can see if I get the same problem.
You can also check in the browser with F12 if you see errors there. Check the Console tab and try to upload to see if you get a message there. You can also check the Network tab and then try. You should see the request in the list. Select the request and check the Response tab. Maybe there’s something useful there.