The answer to your question has been posted 3 times in the last 50 posts. But to sum it up for you, GPU has overhead loaded into it. Either put a 6 or 8GB card as GPU0 or use on-cpu video.
I get the problem even when disabling the first GPU. The overhead is a poor excuse for a miner that is simply outdated. His Zcash miner is even more outdated.
Are you bitching in his thread that he's abandoned for almost a year that he's not supporting the software? Several people have already mentioned his last post date. He's still raking in thousands of dollars every day so "poor excuse of a miner" when over 50% of casual miners were using it at times seems like sour grapes.
The miner works fine when a 6 or 8GB card is placed in GPU0 position. Anything card that doesn't work past that, well that's the whole point of a DAG loaded into memory or Dagger Hashimoto would have been dead way back in 2017 with ASICs.
Go try Phoenix miner.