Upgraded from CRM 4.0 to 2016 (on-prem) ... now get script errors in Contacts

Particularly in Contacts ... when in "CONTACT" 'view' things seem to be okay, but when changing to "CONTACT:Information" in any contact, the script errors start (even when just changing to this view).

One of the scripts for this record has caused an error. For more details, download the log file.
ReferenceError: 'crmForm' is undefined at Form_onload (crm2016.domain.local/.../Contact_main_library.js)

Click OK. All good. Changes can be made to contacts, and mostly there are no issues. Create a new contact, and get the same script error again. Switch from CONTACT to CONTACT:Information view ... and script error appears.

The downloaded log file contains:

ReferenceError: 'crmForm' is undefined
at Form_onload (crm2016.domain.local/.../Contact_main_library.js)
at eval code (eval code:1:1)
at RunHandlerInternal (crm2016.domain.local/.../ClientApiWrapper.aspx)
at RunHandlers (crm2016.domain.local/.../ClientApiWrapper.aspx)
at OnScriptTagLoaded (crm2016.domain.local/.../ClientApiWrapper.aspx)
at Anonymous function (crm2016.domain.local/.../ClientApiWrapper.aspx)

Thanks,

Jason