Yes, your function as filter should always return value. If there is nothing to change, then return unchanged value it got as input parameter.
500 is not a problem. There are installations with thousands posts and/or pages, which work fine.
Try to turn ON wordpress debug mode with writing to file. Add this lines to wp-config.php for that:
define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
WordPress will write all error messages to wp-content/debug.log file in this case. Reproduce the error.
Look if ‘user-role-editor-pro’ is mentioned at the error messages.
Another variant for investigation:
Strange, that out of memory error is raised at different times from different sources. Can you try to deactivate all plugins except URE and test again. In case error will go away, activate plugins back one by one until error will return. Just in case some other plugin eats to much memory.
