Yes. It is now working.
I was able to fix the problem by going into the form editor, selecting Settings, Spam Protection and Security, and disabling the Anti-spam Protection. It was set to on by default, along with my Captcha, which seems to be working properly. Apparently it is this Anti-Spam feature that caused my form to fail, and there must be some kind of bug in the feature that prevents it from operating properly on my web site.
It would be nice to have this feature turned on, but since it is making my form fail, I guess I’ll have to live without it.