We've recently bumped up against this issue during our tests (CRM 2015 Update 0.1). There are plenty of answers online that mention that this error is prevented by disabling Activity Feed Rules and that has worked work us. However, I want to know how and why this issue occurred and what we can do to prevent it without disabling the rules.
When creating a Case, we have a synchronous workflow running that creates and associates a related record (N:1) and on create of that record, populates a value back on the Case.
When the Case is saved, the error mentioned above is shown. The weird thing is that this won't occur when opening a new Case form and filling in the required details and saving. But, when I'm on a Contact there's a subgrid for related Cases based on a custom relationship. When creating a Case through that subgrid and saving, the error occurs.
I've racked my brain trying to think what could possibly be tripping out the Activity Feeds. I've taken a look into the assemblies with ILSpy and the logic doesn't exactly seem complicated and the data I'm providing doesn't look like it would cause an exception. What's also strange is that this occurs on our Managed Test/Production environments, but not our Unmanaged Development environment.
Where should I go from here? Is there another cause that I haven't thought of or is the correct fix in this instance to always just disable the Activity Feed rule and forget about it?