Job/Task control runs jobs as asynchronous when synchronous is enabled.

Conditions for this particular error in my case, so far:

Multiple jobs chained together using Job/Task Control.
Sub jobs run asynchronously even though set to synchronous execution.

I have chained jobs together like this for several years. Only recently did it start to have trouble.

I am on version 8.4.1 build 1485

It's a known issue in 8.4.1 that I am patiently awaiting a fix for in 8.4.2..

We had the same problem..