That was my suspicion. Could be something with the access rights? You can try to start the application with administrator rights. Right click the Hos.exe and use "start as administrator". You can also try to find the restrictions on the savefiles.
C:\Users\your_user\AppData\Roaming\Godot\app_userdata\High School of Succubus\saves
Rightclick save folder or a special save file -> use option -> security -> (and optional) extendend
At my system the whole folder is allowed for system, admin and my username. My savefiles in this folder have the same settings.
I also tested an older save from version before on my own. I have no problems to load this older version. So looks like no bug in HOS coding?
Edit: But could be also a problem, when loading savefiles from older versions, as i said in other postings here. You can´t control it. How old are you savefiles, was it one version before, two version before and so on... There is a limit and also a risk of stucking or crashes when you load saves from versions before.