Don´t try to remove opcache on Debian 11. There is a mandatory dependency for php7.4-fpm and other packages that are required for Grommunio to work.
BTW: same is true for the upcoming Debian 12
Instead, you have to deactivate the opache module in the PHP installation.
This can be done with the following:
You are only removing links with this, so you should easily be able to re-create these links in case the bug in opcache gets fixed.
Don´t forget to restart php7.4-fpm afterwards.