Nextcloud version (eg, 20.0.5): 20.0.9
Operating system and version (eg, Ubuntu 20.04): Ubuntu 18.04.5
Apache or nginx version (eg, Apache 2.4.25): apache 2.4.29
PHP version (eg, 7.4): 7.4.21
Many PHP error messages in log: Module 'gmp' already loaded at Unknown#0
Not all PHP modules are loaded as separate modules and are part of the PHP base binary. In this case the module is loaded without a separate load statement/configuration.
If in addition a separate configuration file exists, e.g. /etc/php7/fpm/conf.d/gmp.ini, the web server tries to load the module twice which can cause these kind of messages.