I ran into a situation yesterday where a customer was seeing this very scary error message:
This is very odd, considering they were already logged into Dynamics CRM 2016 Online and everything was working – until it wasn’t.
After a bit of digging around, I ran into this forum post, which outlined a similar error.
One short screen-sharing session later, and we had the problem solved.
It turns out there was a workflow assigned to a custom entity and the owner of the workflow was a contractor who had recently been disabled. Since he was disabled, his Dynamics CRM license was no longer assigned, hence the error message.
While we did not do so, I think if we had clicked the Download Log File button and reviewed the actually error log contents, it would have told us the exact error message.
As I mention in my Dynamics CRM Deep Dive: Administration book, what I usually do is create a CRM Administrators group, assign this group the System Administrator security role, then add administrators to this group.
I then assign things like personal views and processes to the group instead of being owned by individual users. This practice would have prevented this error from occurring.