admin-ajax.php 403 Forbidden error in dev tools

Hello!
We have just switched to a multisite wordpress in order to have two websites (different languages) and suddenly realized we have a few problems editing our websites:
– can’t add new product and post categories any more (but can add new tags)
– can’t insert media in blog posts (but saving the posts themselves works fine)
– possibly other problems that we have not encountered yet.

Running dev console shows this error repetitively:
POST XHR https://joahbox.com/wp-admin/admin-ajax.php [HTTP/1.1 403 Forbidden 486ms].
This error is not triggered by any specific action (clicking) but keeps throwing itself as long as you stay on that dashboard page (i.e. Add new blog entry).

I have run all the updates. I have no security plugins installed, although have a cache plugin (which i also disabled but to no result).
Everything worked fine before switching to multisite.

Would anyone have an idea what I should look at in order to find the issue?
Thank you so much in advance!
Anne