When the KB that has Dynamic Forms Module enabled also has GAM Security, WorkWithPlus stores the user who filled out the form. This is done with the Transaction WWPUserExtended, which extends the GAM User and automatically syncs when a GAM User is created or updated.

When you enable the Dynamic Forms Module in your KB, you need to impact metadata (WorkWithPlus Menu > Impact Metadata for Web and Native Mobile) to create in the table WWPUserExtended all the users of the application that were already created.

If you do not do this, the following error will appear at the moment of filling a dynamic form: 

Error loading new Form instance (No matching 'WWP_UserExtended'.)