Thanks for your input.
I am in discussions with both our service provider as well as Marketo support.
There seems to be some inconsistances in our testing of this issue.
I have recently been invited to events from Marketo as well as from our service provider Datarati, they both used automated .ics files for their calendar invites.
The Marketo event opened perfectly in Outlook where as Datarati's and mine did not, if it is an issue with Outlook stability wouldn't it crash regardless of what invite was opened because the process is essentially the same (assuming Marketo use Marketo 🙂 )?
This leads me back to thinking it is something to do with the actual .ics file Marketo produces, there must be some disconnect here.