Hi Rolf,
Do you know what version of PHP you are running on the server that is hosting the gallery?
The PM Selections export gallery was written for PHP version 6. There have been reports of it not functioning properly on PHP 8. We plan on addressing this issue as soon as possible.
If the server is running a PHP version less than version 8, then the server error logs should have more information about the error. The server error log file location can vary depending on the server configuration, so you may need to contact the server admin for assistance in obtaining the server error logs.
-Jerry