I also met the same problem when I upgraded to the version 5.6. After checking the source code, I believe the problem comes from the new functionality of state tree which is called “Scheduled Tick Policy”.
If you have the tick task in your state tree, you need to set the “Scheduled Tick Policy” first. Although the Default option and the Allowed option both set the tick policy to true, you also need to set the Custom Tick Rate and give it a reasonable time. Otherwise, the tick task will be automatically shut down because of the setting check for the Custom Tick Rate.
So maybe you can check all these settings and see whether the state tree tick tasks work as usual. Hope these information can help you solve this problem.