Same. When the Support category is selected from the dropdown, the GET request for the Support category form template JSON file throws a 500 error in my developer tools console.
I'm also experiencing this. I'm linking to an upstream Discourse pull request that might be related:
Edit: For context, Joplin's forum uses the Discourse platform. If I attempt to modify the support category's template, I get a "500 server error" page. The linked pull request states that it fixes a 500 error related to topic templates. Since Joplin uses topic templates for the "support" category, I'm hopeful that the linked pull request will fix the issue. (Further context: The pull request is a proposed change to Discourse).