Hi Peter,
it's weird becuase I use both Convert Forms and SPPB for ages, last time 20min ago, really.
- As first test try to publish that module in template module postion "content-bottom".
- Then as 2nd test, Disable compression (pagespeed) that you have and test again, becuase in many cases this kind of compressions may damage code as well.
Do you have testing subdomain, where you have only SPPB + Convert Forms and you can import the same form and test it there.
Yes, Support from Tassos may be useful as well, becuase he knows his extension(s) better than I can.