MPLAB ICD 2 has been asked to perform an operation while it is busy servicing a prior request. For example, you may start a program operation and then attempt to step before the program operation is completed.
Close the SFR or GPR windows. This situation will not normally happen as the MPLAB ICD 2 user interfaces will be disabled while servicing requests. The most common occurrence of this error is when MPLAB ICD 2 is being restarted (perhaps because a project has been opened) as both a programmer and a debugger simultaneously.