Hey there,
as I wanted to keep my nextcloud as slim as possible I disabled user avatars by adding 'enable_avatars' => false, to the config.php.
First I realized some errors in spreed app, where the browser console finally lead me to the reason of missing user avatars:
Some hours ago I wanted to share a folder with another user of my nextcloud instance, but typing it’s user name did not show up the user “button” to invite. Having a look at the browser console it again leads to missing user avatars, which seem to be expected by the share feature to pop up the users while typing their name inside the mask.
So to speak generally, if I am not doing something wrong, some apps and core parts of nextcloud expect user avatars to be there and throw errors if they are not. As there is some official option to disable avatars through the config.php, there should be at least hints about broken apps/features or better some alternative behaviour, if user avatars are disabled.
Removing the ‘enable_avatars’ setting from admin manual and better also as working setting, to ensure avatars always available, would of course also be a possibility ;).