Note: Some of these steps may be superfluous, but I was able to reproduce the issue consistently by following these exact steps and I didn't feel like testing over and over until I figured out how necessary each step was.
Browse > Collections > New Collection
Fill in required information
For "Type of challenge, if any" choose "Prompt Meme"
Make sure "Sign-up open?" is checked
Under Request Settings:
Check both Required and Allowed for Title
Uncheck both Required and Allowed for Description
Require 0 and allow 1 for Fandoms
Require and allow 0 for other tag types
Follow the Prompt Form link
Fill in just a Title for your prompt
Follow the Claim link on the prompt
Now follow the Fulfill link on the prompt
A 500 error. This error is happening because the prompt's description is nil, and it's trying to convert nil into a string to fill in the author's notes:
ActionView::Template::Error: no implicit conversion of nil into String
The post new form should load with prompt-related information filled in
For a workaround, users can go Post > New Work instead.