Hi Halicate,
I love your suggestion. I should have thought of that, but hadn't. Unfortunately, though, I got "too many redirects" and "Bad Request Error 400" again on a copy -- and on a copy of a copy of the Sheet I'm working on.
Your suggestion prompted me to try to launch Apps Script from a new Sheet, but I got "too many redirects" and "Bad Request Error 400" again in a new Sheet.
I had a lot of windows and tabs open in Chrome, which I figured could pose a problem, so I've relaunched Chrome and got "too many redirects" and "Bad Request Error 400" when trying to launch Apps Script from the Sheet I'm working on -- and also on a fresh Sheet.
This can't be a problem with Google's servers at this point. It's been days and days of getting this same error.
I must either be "doing something wrong," or not doing something I could be doing to solve this.
Looking forward to more ideas! :)
Matthew