Adding multiple people to channel

Hello there,
when I’m trying to add multiple users to a channel no one or just one user will be added.
I’ve already checked the troubleshooting guide, but it wasn’t very usefull.
I hope, that someone can help me.
I’m running Mattermost 5.24.1 with the testlicense.

How are you adding the users to a channel and do you see any log errors?

I click on “Add Members” then I choose two or more users (Mattermost is connected with our AD if it’s important) and I click on “Add”.
It loads for around 20 seconds and after that nobody (or one user) has been added.
The log just says, that these users couldn’t be added with an http error 500 code.