I was getting the 256 error when installing as well as other folks in this thread, and my issue and solution were...interesting.
After someone suggested that their installer wasn't downloading properly and they got it manually, I did the same, and found my router was blocking the URL found in the Lutris PSOBB JSON. I don't know why my router dislikes the URL, but making the link https rather than http fixed that.
So I downloaded the JSON from the Lutris CDN and edited the link to the file to be https rather than http, pointed Lutris at it rather than searching via Lutris, and it worked just fine!