Hi @Frank Martin ,
This is the list forms schema rather then form tag in Master Page:
And this solution is working for custom list form page, for OOTB display form, it's not suitable.
For the error message "Error during serialization or deserialization using the JSON JavaScriptSerializer", I suggest you could check out if in the list, there is lookup field which lookup a list with large items, try to reduce the lookup list cout or you can try other field type like Choices instead of Lookup dropdown column to overcome this issue.
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