If you login via SSH, you can look up the current permissions for example by using ls -l. I don’t have humhub, but to get an idea,
$ ls -l
dr-xr-x--- 14 element www-data 4096 Feb 18 13:33 element
drwxr-xr-x 2 root root 4096 Oct 3 13:13 html
drwxr-x--- 17 opensondage www-data 4096 Dec 15 18:54 opensondage
drwxr-xr-x 5 wordpress wordpress 4096 Mar 17 19:49 wordpress
The directories are owned and writable by various system users (the first rwx behind d). They are accessible by their groups, but read only (r-x, read and execute/enter directory).
You could have a look who owns the directory /var/www/humhub/uploads ; I would expect it is set correctly by the Yunohost installation process, also because you are able to upload small files.
In php.ini there is a setting that sets the size for file uploads, the first result that I found is at Stackoverflow:
There are quite a few php.ini files on my Yunohost, because different versions of PHP can coexist on the same system. You’ll need to find out which PHP is used by HumHub, and change the corresponding php.ini (or change them all), for example /etc/php/7.3/cli/php.ini
the file you’re looking for is located in /etc/nginx/conf.d
it will be named the same domain name followed by .conf
you’ll add client_max_body_size 1024m;
like explained here : What is client_max_body_size in Nginx
be careful not to change anything else