A CLR error occurred while invoking the scheduling engine

While running Master Planning in AX2012, a lot of issues can crop up and one of the messages we see in the info-log is "A CLR error occurred while invoking the scheduling engine.". There are various causes for this issue. Following is a list of things to check in case you run into the same issue. 

• Check the "Enable the hot-swapping of assemblies for each development session" is turned off in the server configuration utility. 

• If the info-log message includes a message such as "The given key was not present in the dictionary" and the stack trace points to "Microsoft.Dynamics.AX.Planning.JobScheduling.ResourcePrioirtySolutionComparer" then the issue is related to a route operation that doesn't have any requirement defined. Following are the steps to fix this type of issue.
• From the master planning info log, look at the associated order number for the error. 
• From the planned order form, search for the order number and open it. 
• Click the routes button on the order to see the associated route. 
• For each operation in the route, check that at least one resource requirement is defined.
• If you find any operation without any resource requirement, open the associated route and define the resource requirement. 
• Re-run master planning and that should resolve the CLR error.