First, create a tag set with more than 20 tags of one type, e.g. 21 characters:
Browse > Tags > Tag Sets > New Tag Set
Fill in a title
Enter more than 20 tags of one type in the appropriate field, e.g. 21 characters in the "Add Characters" field
Second, create a gift exchange that uses the tag set and is open for sign-ups:
Browse > Collections > New Collection
Fill in required informat
For "Type of challenge, if any," choose "Gift Exchange"
Check "Sign-up open?"
Make sure the type of tag you used in your tag set is allowed in requests or offers, e.g. if your tag set uses character tags, you should be allowed to request at least one Character
In "Tag Sets To Use," fill in the name of the tag set you created
Now try to access the list of available tags:
Follow the "Sign-up Form" link in the sidebar
Use the "List [X] [Tag Type]" link
A 500 error:
It should work.