Smart Proxy communication problem after upgrade to 3.12

Problem:

After the upgrade to 3.12 (using foreman-installer), the puppet smart proxy does not update its status anymore (the “pending” symbol keeps spinning).
I also cannot add/ remove puppet classes to/ from hosts. I can see them, but clicking them doesn’t do anything.

Expected outcome:

Expected to see the current status of the smart proxy.
Expected to be able to add/ remove classes.

Foreman and Proxy versions:

Foreman: 3.12
Smart proxy: Cannot see status and version anymore

Foreman and Proxy plugin versions:

Distribution and version:

Ubuntu 20.04 LTS

Other relevant data:
I am able to import new classes to the environments.

Update from the author:

The error was client-based and depended on the used browser. Can be closed. Thanks!

So this is now resolved.