I also have a custom GPT I have been using last year that now just replies with “I can’t fulfill this request.” no matter what I do. It’s due to more and more restrictive policies by OpenAI. You now basically have to search your whole prompt for anything that a) could be mildly offensive to anyone (which includes very tame topics) or b) in any way contradicts the new guidelines. For example telling GPT to not shorten anything for brevity can turn up this result, since apparently OpenAI does want to waste as many “easy” tokens as possible.