Had this same problem. Started to think like the others above, "Oh no i cant mine anymore as dag files too big for my GPU" etc.

Take DAG file size, multiply by number of GPUS and set virtual memory to this size.

For me;
12x3GB GPU's
12x2.3GB = 27.6 GB so set virtual memory to 32GB (bit of overhead for DAG file size increases).

Done.