Nextcloud version : 20.0.8
Operating system and version (eg, Ubuntu 20.04): Ubuntu 20.04
nginx version: nginx/1.18.0
PHP version: php-fpm 7.4
The issue you are facing: Many buttons on the UI are invisible
Is this the first time you’ve seen this error?: Yes, with nextcloud 20.0.7 I did not have this problem
Steps to replicate it:
from the browser net panel it doesn’t look like some SVG is missing or something like that
Maybe desactivate php-fpm ?
I’ve done this and my similar problem was resolved.
More details here (but in french).
The only difference I did after the update was inserting those values: Server tuning — Nextcloud latest Administration Manual latest documentation
I reverted to default, it still does not work…
edit: I disabled ALL the apps and now the icons are back. Now I need to find which one causes the problem…
When you’ll find wich one is the cause, tell us 
I had the same problem installing the Bookmarks app (before it was not installed). All users lost icons. Removing the bookmarks app restored icons to everyone.
