Unfortunately, the same result. I'm going to try backing up the database and the config file, and doing a complete recreation of the domain, in case something got messed up there somewhere and I'm just overlooking it.
Will return shortly (hopefully, anyway) with an update.
<<follow up>>
Sometimes it's easier just to wipe out everything and start over. My wife keeps telling me that its not how I should be avoiding my life responsibilities, but if it works...
Doing a database (and config.php) backup, completely removing the account from WHM, and then doing a fresh recreation (this time remembering to disable phalcon5 and psr), then restoring the 2 files, seems to have sorted out the prinsfrank error. Let's hope it stays gone, because my WHMCS license expires in 3 days, and that $40+ a month for something that used to be $15 really hits hard.