AxiosError: Request failed with status code 400 when creating a room #422

Describe the bug
I installed the bbb addon manually and built it with "make build" - nodejs and npm are both installed.
I set the BigBlueButton URL and Secret (Scalelite behind it) and when trying to create a room, I get this error:
AxiosError: Request failed with status code 400
Looking into the browser console, I can see this status code comes from apps/bbb/rooms, so I tried opening it manually.
When opening it manually (nextcloud.xxx.com/apps/bbb/rooms) I got the following screen:

Access denied
CSRF check failed

To Reproduce
Steps to reproduce the behavior:

1. Install addon manually according to README
2. Set Secret and URL
3. Try to create room

Expected behavior
Room should be created without a error.

System (please complete the following information):

• Thorium / Firefox - both didn't work
• App version 2.9.1
• Nextcloud version 32.0.5
• BigBlueButton version 3.0.21 / Scalelite 1.7.1
• Is your BigBlueButton instance working with Greenligh? yes
• Did you enter your BBB API url and secret? yes
• Are you using a load balancer, like Scalelite? yes

Nextcloud log:

There are no error messages in the nextcloud log.

Javascript log:
Failed to load resource: the server responded with a status of 400 (Bad request) Api.ts:122
POST https://nextcloud.xxx.com/apps/bbb/rooms 400 (Bad request)

Additional context
I also installed this addon on a fresh nextcloud installation - on there everything works as expected