Did you rinse after backwash?
Foaming is almost always organics which chlorine can take care of. I'm still stuck on organics are at work. Don't panic. We'll get this.
Did the Pool Builder add any algaecides or clarifiers, or any other pool potions? Do you have a list of them?
Let's do a proper OCLT tonight...I'm
almost at the point of telling you to start a slam without an OCLT, given the foam and the rapid rise in the filter pressure.
Read the SLAM process and follow it. Link-->
SLAM Process
Let your FC come down below 10, adjust pH per slam article and let's get this slam out of the way. Do the OCLT too tonight.