V1.0 broken with CF 5.4 and above

I periodically require a form that does calculations on a site I maintain. I have the PRO version of this plugin, which “did the job” – until this season.

It looks like this plugin works fine with Contact Form 7 up to version 5.3.2, but breaks with 5.4 or above. This update occurred in February, and its now 7 months later, and no new released version. No apparent way to contact author, no notes anywhere about this – this plugin appears to be a time sink dead-end.