I am facing an error in Nintex Forms. I have problems finding the cause as the form is pretty massive (formula, lookups, rules).

Error message in English (translated): "Unable to get property '_ObjectType_' of undefined or null reference"
Error message in German: "Die Eigenschaft "_ObjectType_" eines undefinierten oder Nullverweises kann nicht abgerufen werden.
I found a solution for a similar looking issue, but it seems not to be this issue as there is no "FormsPart.aspx" referenced in this case.
As the form is pretty massive I am not able to remove all columns and rules step by step to get the reason for this message. Is there a better way of debugging?
Version is 4.3.1.1 for SharePoint 2016 on premise
Nintex Forms Standard
SharePoint 2016 May 2018 patch level