All,
I made a copy of hell-forms.hs from yesod-form package, just for doing some experiments.
When I run it via the main function, the server is doing well e.g. on port 2500.
HalloWelt.hs is here.

But when I try to run that program in bash console via wai-handler-devel - being in the directory where the HalloWelt.hs resides...

  wai-handler-devel 2600 HalloWelt withHalloWelt

I get...

Attempting to interpret your app...
Compile failed: NotAllowed "module is not loaded: `HalloWelt' (./HalloWelt.hs)"

What could be the reason for that?
Thank you for any hint -
Best regards
Hartmut