Thanks for the response, Takayuki.
I did deactivate all plugins (other than CF7), and I use the K2 theme which is pretty basic, and nothing fancy (in fact I think it's pretty similar to the WP default theme itself). I'd prefer not to have to use the WP theme anyway.
I first disabled the plugins one-by-one, then I disabled them all and the problem stuck.
I don't want to disable the Ajax feature, I just want it to work like it used to; I have the common error of the spinning arrow.
I've followed every piece of advice you have kindly given on your FAQs, and yourself and others have given here on the WP forums; and this temporary fix was the only thing that worked for me at this time.
I've reversed it following your post.
I do appreciate you must get asked a lot of the same questions, and you do it for voluntary donations. This kind of thing isn't my area of expertise, and I'm open to your thoughts on a possible solution.
One of a few different CF7 forms on my site can be found here.