Possible reasons

• mod_secure: Please ask your provider about possible issues with that. Doing this solved 99% of these issues.
• Error 500: PHP memory size too low. I have had good results with 128M, depending on the size of your images.
• Error 500: weak server. Every thumbnail request is a PHP request. Some servers can’t handle the necessary amount of parallel PHP requests. You can switch to pre-generated images in the components options. Don’t forget to use the thumbnail creator to generate the necessary thumbnails for all your existing images.
• Do not use special characters such as umlauts or + in your file names. Make your file names safe for the web.
• Install GD library for image processing. Double check after you switched to another PHP version!
• PHP should have write permission to /images, /cache, and /logs
• Error 500: PHP should be able to execute the script /components/com_eventgallery/helpers/image.php in order to display thumbnails. If you can’t/don’t want this, use pre-generated thumbnails instead of dynamically generated thumbnails.
• Try to use the configuration Use Rendering Fallback to verify rendering thumbnails work. Ask your provider how to fix the permissions for components/com_eventgallery/helpers/image.php.
• Log file is too large: delete /logs/com_eventgallery.log.php
• Sh404SEF: you might want to configure the Flood Protection to prevent images from randomly disappearing.
• The PHP method fpassthru is disabled. This method is necessary to deliver images from the server to the browser.
• Google Photos API: thumbnail URLs are valid for up to 60 minutes. Then they need a refresh. Make sure your Google credentials are still valid so that Event Gallery can retrieve new URL data.