Hi @Anonymous ,
Regarding the issue that you mentioned, I agree with @justinburch 's thought almost. Please disable the Custom Errors option in your Portal's Administrator, then you would see the details info about this error.
Have you tried to create a new workflow for your Entity, then bind it into your Web Form Step again, then check if the issue is solved.
In addition, also please make sure that the Web Role assigned to you has sufficient permission to access the Entity resource related to your Web Form.
Regards,