A 12 year old forum post pointed me in the right direction. I had to adjust my Maximum number of POST request parameters to match Production and everything seems back to normal. Any idea why this would have been reset? https://community.adobe.com/t5/coldfusion-discussions/tomcat-exception-cannot-call-senderror-after-the-response-has-been-committed/td-p/4380613

Were any other changes made to the server? It's the same server as before, right? Kinda weird that the error message you got means what it apparently means.

That's the thing. I didn't change anything - it just stopped working. It's like the CF Admin had a setting change that I wasn't aware of.

So no server or java updates? That is weird.

No. CF is several updates behind current. Haven't updated Java since I got this laptop and did the initial CF 21 install (earlier this year). And the "server" is a Windows 11 laptop for DEV purposes.

Could it be that you hadn't visited the page that generates the large number of POST request parameters till now? I recall running into this same issue many years ago on an old legacy app. It was only tested on a small subset of data in Dev, but in production, the users could dynamically add additional fields and this quickly surpassed the CF Admin limit.

I suppose it's possible I hadn't done that on my local DEV since I got this laptop earlier this year...