The AA game engine (which hasn't been updated or supported by the US Army in years) failed to release memory from the old map/reallocate/ran out of memory for the new map when switching maps. Try waiting a minute, closing anything else you have open and try again. If it happens all the time you can try restarting your computer, shutting down unnecessary programs, add more memory to your system, switch to SSD drives instead of regular rotating hard drives - but you'll still have the problem from time to time.