Hi,
We have a customer who was upgraded to 2016 yesterday (8.0.1.344). They are Online. Since the upgrade they are unable to set the Regarding field on mails that have been received into an incoming mailbox as the record is read only. See below for the error they are getting. This error is even occurring using the Sys Admin account. I found a blog post which describes the same issue occurring after an upgrade but no solution was given (https://community.dynamics.com/crm/f/117/t/201686). Any help is greatly appreciated.
Simon