When adding a issue type scheme, and saving it, the selected issue types get sent in the URL of the request. When the amount of issue types is large, the length of the request can increase to problematic lengths. For instance, Apache will only accept 8K characters in the URL request, and even after updating some of their configuration parameters, the request is rejected. Also, browsers have limits to the length of the URL they can post.

Basically, the selected options should be sent as part of the post parameters, and not in the URL.