Object.dat Fix Tool

Extract the zip file and run UnturnedPreviewObjectDatFix.exe in the zip file to open the tool.

Type in the full path to the dat file, press enter, then confirm the path looks right and type 'Y' for yes and press enter.

Wait until it's done, then go to the folder where the file was and there should be a new file called 'Fixed Object.dat' next to it. Rename your broken one to something else and rename the fixed one to just Object.dat and boot up without the preview branch installed.