I am very sorry, but this is utterly depressing.
It worked twice and I received contact emails, and then it suddenly and for no reason stopped working on a third attempt.
I eventually decided that perhaps that particular contact form had gotten corrupted with so much inserting and removing of code that I decided to build a completely new form. I then went through a phase where I got a constant debug error message above the form saying something about "local host not existing". I regret that I did not write down the exact error.
Then next I went through a phase in which it refused to authenticate, telling me that either password or username was incorrect when they were identical, and so I rebult from scratch again.
And now … I am getting 500 internal server errors.
Errors do not remain consistent so it is difficult to figure out what is wrong.
The only thing I am coming up with is my fear that incompatibilities with that PHP version are in play.
I need to eat.
This is driving me quite insane.
I will try again after I have rested, eaten, rebooted computer yet again, and other such things.