Contributor guidelines page here has a link pointing to ASF Jira. The link targets a filter, owned and published by Daniel Kulp. Something in the data structures appears to have changed over the course of time and Jira is erroring out. This could be a turn-off for potential new contributors.
To fix there are two ways.
1. If this filter could be fixed by its owner, thats the easiest possible option
2. Alternately, someone with access to ASF Jira to create filters should create an easy to pick filter and the link the the filter should be updated at <root>/docs/user-manual/modules/ROOT/pages/contributing.adoc : Line 9