The error means that CORE was unable to complete your request.
There are two reasons the error might occur
1) Your request required a large amount of data and it could not be completed at this time.
2) Something is broken.
As suggested by the error message, you should always retry whatever you were working on. Sometimes waiting a few minutes will provide CORE enough time to complete your request.
If, after trying again, you are unable to complete your task, please send a ticket to the firstname.lastname@example.org. Be sure to record the unique reference number and describe what you were trying to do at the time of the error. Without the error number we won't be able to review it for you.