@meme/Oldpilot - Oldpilot reported issues with the 905 beta as well and sent me an ePub at the time that worked perfectly fine for me. I am inclined to believe it is some other software/Qt conflict on their machine of some kind, particularly since they claimed continuous crashes with new epubs etc.
My suggestion at the time was for them to try Sigil on another machine, a virtual machine or safe mode with the same epub inputs etc. Something Oldpilot said they would try and that is the last I heard from them.
So unless this time around there really is a problem with a specific ePub (always possible, but based on the previous experience I would suggest unlikely) the onus is really on Oldpilot to try a different machine and see if they have the same issues. Because *nobody else* has reported the same issues on Windows systems. Whether Sigil 0.5.3 works or doesn't has little relevance, as so many components and code have changed. It might be some antivirus type conflict, hardware level like the historical Wacom issues, graphics drivers or any one of a number of other environmental issues. If we can't replicate it, we can't do anything about it.