Vanilla 2.8 - comment field / new discussion field gone

Hey guys,
i managed to install Vanilla 2.8 and migrated all my data from phpbb3. Just one problem that i have is the following.
Whenever i try to open a discussion or leave a comment to an open one the field immediately disappears. I can see it for a second and then it's gone. I can't really describe it. See screenshot attached. It would be great if anyone can help me with that :)
Thank you!
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Comments
I can add one more thing. this only happens when i use the "post format" set to "rich". if i set it to "text" it stays.
There is a similar discussion already:
The problem seems to be that some routes do not work as expected. You might be able to find out by using the web inspector tools of your browser and looking at the network protocol.
Also looking at the webserver error logs could give more information.