I don't know exactly why but I solve this error by completely deleting the saves from the game and those from the rempy folder in AppData\Roaming.
I wanted to start the game from scratch and realized that I no longer had that bug.
In short, if you want the game to work with the itch app, the steps you should follow are:
1st Install "Changelog.txt" in the app (Not the game), this will create an installation folder.
2º Once "Changelog.txt" is installed, locate it in the game library, right click on the game image and select open folder in explorer.
3º Download the game from any browser (the app will always fail).
4º move and unzip what was downloaded in the previously opened folder.
5º Itch.io app should be working now
I think it's a mistake by the developer when configuring the behavior of the itch.io app.
You can try downloading the game directly from a browser.
A very typical trick for this type of failure and for it to work in the app is to have the game previously installed and download the update through Chrome or others and once downloaded change the content of the game installation folder with the downloaded one.
I know that Steam is a delicate tricky subject and I am also not an expert on the subject but I have heard that many games that were banned on Steam nevertheless entered GOG. Usually on GOG it works by separating the game from the main "content" in a free patch similar to how Kagura Games does as well. I hope the piracy situation eases up. Your game is one of the best purchases on this platform, you are an artist.
For now don't use the itch manager until the developer adapts it (he recently commented that he contacted itch.io to increase the limit of 1GB per link). Manually download each file, for example from the email you received for the purchase you should have a download link. Download the 4 files (.exe .001-.003). If they are all in the same folder, run the .exe and wait.
If you want to use the Itch.io app you can cheat the program and install from the manager only the SlaveBar v1.3 M.exe (this way itch will create a specific folder). Move everything you unzipped earlier to this folder and you should be able to run the game from the itch.io app by selecting the correct exe.
The folder structure for the manager should look something like this:
-.itch*
- New folder**
- SlaveBar v1.3 M***
* Folder auto created by itch.io app
** Here pull the exe and the rest of the parts
*** Here are the game data
I hope this helped you in some way and sorry that English is not my first language.