When setting up a complex race you can quickly make a tiny mistake and accidentally create a Circular Reference. For example, the definitions T1:=T2+1 and T2:=T1+1 create a circular reference. If T2 has been updated, T1 needs to be updated. In return, T2 needs to be updated and then T1 needs to be updated again. This will create an endless loop so that the system would never stop working unless it detects the circular reference.
The SportsEventServer detects even long complex circular references and then refuses the calculation since something must be wrong in your definition. If your event has circular references, these will be listed in Overview->Technical Information. If something in your event file does not work the way as expected, it is a good idea to check for circular references here.