Error 500 on Print/Merge Document page when doing Thank-Yous
General recipe for Errors like that is to find a clue as to what's going on in the log files: first two places to check:


check the CiviCRM log in /files/civicrm/ConfigAndLog/
check your php and/or php-fpm error log (location of that depends on your install one possible option is: /var/log/httpd/