Turn notices display off if you don't know what that means. That's probably what "Camille" was refering to when she said "
there is still an error in the script. please refer to their technical support to find the fix for this script". Notices display is a developer-only feature that you shouldn't have enabled in the first place. There is nothing wrong with the script, as it is being used on thousands of webservers. The release cpg1.4.26 works with your PHP version.
An 500 internal error is not a syntax error of a PHP script. It's an unhandled error that can only be tracked by looking at the server's error logs. Usually it is related to resources consumption, i.e. the script runs into a time-out because of restrictions imposed on your webserver by your webhost. As Camille failed to ask at which time you got that hard error she can hardly tell what the error message actually was about, as she would have needed that piece of information to look it up in the log. She hasn't asked neither for a coppermine admin account, so she can not havae reproduced it. In the worst case, she just has seen the error message "You don't have permission to access this page", which is to be expected and concluded that this error message is the cause for your trouble. Ask your webhost to do their homework and take a look at the relevant section of the webserver's error log. You're welcome to refer to this thread. Imo you have just got a canned response from a junior supporter at your webhost's chat desk.
I have made a test account
User: Test
password: test
You have
not done as suggested: the number of URI fields is set to 1 (should be 0). You have enabled notices display (should be disabled). Do your homework, then try again.
Joachim
P.S. The URL
http://www.circuitfotografie.nl/cpg1418/ is just plain silly. You should rename that folder using your FTP app and then reflect the change in Coppermine's config, as explained in
http://documentation.coppermine-gallery.net/en/export.htm#moving_folder . This is not related to the issues you're having, but just annoying.