The GUI says it cannot reload and I should use fwconsole reload --verbose. This one makes the job without an error, but the GUI is still showing that I have to apply the config.
With Debian there is a genius solution. A second instance of the apache web server can run under another user and also with another PHP version. From the official PHP maintainer there are packages available for PHP 5.6 for Debian 9 and 10. With this configuration FreePBX 14 works without a problem.