I have debian (not raspbian) buster installed on a raspberry pi, and had the same issue with imagick module not loading in apache.
To solve this, what worked for me was to add the following in /etc/php/7.3/apache2/php.ini
extension=imagick.so
Which triggered a php error “Module ‘imagick’ already loaded at Unknown#0”. To fix this, I had to comment out the following in /etc/php/7.3/apache2/conf.d/20-imagick.ini
I made the extension=imagick.so entry in php.ini, because the the error message in nextcloud admin apears. After that the error message was gone. But after a while i recognize messages in cron log:
(I use cron for nextcloud background jobs)
PHP Warning: Module ‘imagick’ already loaded in Unknown on line 0
PHP Fatal error: Uncaught Error: Call to a member function getLogger() on null in /var/www/nextcloud/nextcloud-18.0.4/cron.php:162
In detail: with the entry in php.ini i got no error in nextcloud admin system …missing imagick php module, but after a while an error from cron.
Without the extension entry in php.ini i got the error in nextcloud admin, and cron runs without an error message.
phpsysinfo() gives me allways information about the imagick module, with or without the extension entry in php.ini. So from that side everything seems allright.
I read it. Without - hoppefully, from security view descript in nextcloud forum - because phpinfo() says other - imagemagick php module was gone.
The warning goes away with occ app:disable theming , descript as behind your hyperlink.
From my point, i like imagemagick. The server runs only in local network enviroment, and only known users have the permission to write. This gives me a good overview about security, I think. And so I think I have more negativ effects without imagemagick.
Hopefully the developer find a good solution to that.
I had this same issue on NextcloudPi on the Odroid-HC2 with Nextcloud Version 21.
I first did this (installs for php 7.3):
sudo apt install php-imagick
sudo phpenmod imagick
sudo reboot (this took care of the error)
However, then I got an error complaining about lack of SVG support in imagick.
To fix this error, I used this thread: How to enable SVG for php-imagick and did the following: sudo apt install libmagickcore-6.q16-6-extra