Outlook on domain computers is cycling disconnected/connected. External fine. Error with "MSExchangeMapiMailboxAppPool"

Exact error:

A process serving application pool 'MSExchangeMapiMailboxAppPool' suffered a fatal communication error with the Windows Process Activation Service. The process id was '9020'. The data field contains the error number.

Basically the situation at the moment is everyone who is internal's Outlook is not connecting to Exchange. It cycles between "Trying to connect...", "Connected to Microsoft Exchange" and "Disconnected". one cycle is 20secs or so.

External users are working fine.

Test-outlook connectivity works, test-mapi connectivity works. OWA works. No updates were installed prior to that event showing up in the log and it repeats every 20secs or so. No restart for a few days prior ro issue either. When we restart the server connectivity returns for all users for a solid 5mins then goes back to cycling connected/not connected.

We've tried adding

<add key="HttpProxy.ConcurrencyGuards.SlsByServerGuardLimit" value="2000" />

to the WES web.config file.

We've tried uninstalling the Windows Process Activation Service feature, restarting and then reinstalling it with no change in behavior.

Other errors that we have been getting since this mess started are

Faulting application name: w3wp.exe, version: 8.5.9600.16384, time stamp: 0x5215df96
Faulting module name: KERNELBASE.dll, version: 6.3.9600.18007, time stamp: 0x55c4c341
Exception code: 0xe0434352
Fault offset: 0x000000000000871c
Faulting process id: 0xc078
Faulting application start time: 0x01d1e7c037ee4b5e
Faulting application path: c:\windows\system32\inetsrv\w3wp.exe
Faulting module path: C:\Windows\system32\KERNELBASE.dll
Report Id: 7e171e34-53b3-11e6-80d6-141877366ee5
Faulting package full name: 
Faulting package-relative application ID: 

and:

Watson report about to be sent for process id: 49272, with parameters: E12IIS, c-RTL-AMD64, 15.01.0396.033, w3wp#MSExchangeMapiMailboxAppPool, unknown, M.E.D.T.I.F.F.TextStore.AddText, System.IndexOutOfRangeException, 76cd, unknown.
ErrorReportingEnabled: True 

OS is Server 2012 R2 Std. Exchange 2016 is Version 15.1 ‎(Build 396.30)‎ Multi-site AD, but with only this single Exchange server.

We've contacted MS partner support but they're apparently 4-8 business hours away...

Any help or tips would be greatly appreciated.

Cheers,