lepagegilles Indeed this came up, as I way testing new themes. I installed Trema manually, after the theme was selected, got only white screen. Then forced back Boost through config.php. After deinstalling Trema and removing the theme line in the config.php, now I get 502 Bad Gateway. When that happens php7.3-fpm.log gets the following line written:
[20-Oct-2019 20:00:34] WARNING: [pool www] child 2031 exited on signal 11 (SIGSEGV) after 409.954674 seconds from start
I can not initiate the error deliberately. But once it starts, it persists until I run 'sudo systemctl reload php7.3-fpm.