Hello everyone, HibbiGibbi here!
I love to use UO Steam for my Ultima gaming experience. However I started receiving the subject error message when I got a computer with windows 10. I posted the issue onto a forum for UO Gamers Demise but never received a solution. Then one day I stumbled across a solution that worked for me- with some minor tweaks- on a UO Forever forum. I'm posting a link to both the forum posts just in case players here at UO Phoenix ever have this issue and are seeking a resolution.