It is likely you have reached the 2GB file size limit for 32-bit systems. Check to see if the res file is larger than 2GB. If so, you will need to post process it on a 64-bit system. Unfortunately, there are no other workarounds as it is a fundamental limitation of 32-bit systems. If you are close to the limit, consider writing an Essential results file, which contain only a limited set of variables instead of full results files. To get the full results file from it later, change the Def file to specify full results files, use the Essential res file as the initial guess, and run for one more iteration.