[DiapDealer]

Also consider antimalware settings. Windows' "Controlled Folder Access" is a big offender when it comes to "protecting" Documents & Settings areas from programs it mistakenly deems "potentially unfriendly."

On Windows 10 my personal practice is to exempt programs I trust from Antimalware measures. For Sigil, this includes its program directory and setting a customized temp directory (that my user has full control over) for Sigil that I exempt as well. I also exempt Sigil's preference directory.

I deem those steps safe for anyone to do (but only if they're experiencing trouble using Sigil). If you trust us, you trust us. If you don't, you don't. I also turn off Windows' Controlled Folder Access, but I'm not suggesting that others should take that step. It's just that I find it more trouble than it is worth RE malware prevention.

Also make sure you're not trying to open epubs/documents from synced folders such as Dropbox, One Drive, and the like. These folders often have processes that can partially lock files while monitoring them for changes.