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Which software do you use to find a work item solution (I might've missed it in case you already wrote about it)?
Did you modify the software in any way?
I can't remember having issues to submit the first hit of a work item solution many many years ago when vanitypool was rather new and there were plenty of not terribly hard work items available (GPUs back then weren't as powerful as today's and I only had a barely middle-class one).
It would be against all odds that I was purely lucky that my first found solution to a work item was just one that worked out fine, if there were detected "solutions" that won't spit out the correct public address prefix after combining the keys.
Why would there be something like "1 in 6" chance? That's nonsense in my opinion as I don't see any mathematical proof for such ambiguity. This sounds much like some strange bug in the software.
And I also can't remember any discussion about such an issue in this thread earlier (not that I've followed the many pages of it to the word).