jQueryAjax error preventing some records loading on on-premise 2016

Anyone seen a jQueryAjax undefined reference error in the browser?   It's stopping a number of records from loading up on their entity form.  (The screen remains on the "Loading Business Logic" spinner.)

This is an on-premise 2016 (not sure which update version) that was upgraded from a 2015 org.  

It's currently happening with some Case entity records and with some records for a custom entity.

So far we/the client has tried removing scripts, disabling workflows, disabling business rules, disabling business process flows, creating a new form, and enabling legacy rendering (which apparently allows the form-load to get a little further but still hits the error).   

Will raise with Microsoft but if anyone's hit and fixed or determined the cause of this, please let me know! Stack trace info (anonymised) below...

Thanks,

Mike.

   ReferenceError: 'jQueryAjax' is undefined
   at Microsoft.Crm.Client.Core.Imported.DeferredPromiseHelper.when (crm-dev.some-companys-domain.com/.../global.ashx)
   at ProcessControl.Reflow.DefaultPathManager.prototype.initializeComplete (crm-dev.some-companys-domain.com/.../JsProvider.ashx;ver=-411857949:13599:13)
   at ProcessControl.ViewModels.ProcessControlViewModel.prototype.$6m_0 (crm-dev.some-companys-domain.com/.../JsProvider.ashx;ver=-411857949:16138:9)
   at ProcessControl.Services.ProcessControlInitializer.prototype.render (crm-dev.some-companys-domain.com/.../JsProvider.ashx;ver=-411857949:15563:13)
   at Mscrm.RefreshPageHandler.prototype.$45_3 (crm-dev.some-companys-domain.com/.../JsProvider.ashx;ver=-411857949:2:126854)
   at Mscrm.RefreshPageHandler.prototype.staticScriptsLoadedCallback (crm-dev.some-companys-domain.com/.../JsProvider.ashx;ver=-411857949:2:114502)
   at Anonymous function (crm-dev.some-companys-domain.com/.../JsProvider.ashx;ver=-411857949:2:114329)
   at notifyOnAllScriptsLoaded (crm-dev.some-companys-domain.com/.../PageLoader.js)
   at Anonymous function (crm-dev.some-companys-domain.com/.../PageLoader.js)"