Berzerk2k2: Hmm that's really weird. When I search for the save file location all websites (e.g. PCGamingWiki) point me to the same direction on Windows which is "...\AppData\Local\Loop_Hero". When you have any new info about this please let us all know.
Good news I was able to restore my 17hour save file. For all who might have the same issue missing the loop_save0.bk1 file here is the solution.
Copy the save files you have in a sepparate folder. Delete the Loop_Hero or Loop_hero (thats the folder I found) in the "C:\Users\USERNAME\AppData\Local\Loop_Hero" If you don't find a folder there like I did. Reinstall again before that step.
After deleting the folder start loop hero again. It will create the Loop_Hero folder again. Close Loop Hero and copy your save files into the
Loop_Hero folder again. Copy the
loop_save0.bk. Rename the copy to
loop_save0.bk1. Ignore the warning that the file might become unuasable after the change. Fire up loop hero again. You will find your save restored there. Ignore the warning that the time is not matching. After starting the save file and saving it again the time will be restored.
lkmlkmlkm: A bk file is just an older backup of the save in case newer ones get corrupted, they can be deleted and it'll still work. loop_save0 is the active save, you could try renaming that to something else then the bk file to loop_save0 to make that the new active save.
You can also search your hard drive for loop_config, loop_save0 or Loop_Hero to try and find the save directory.
Thank you too. Maybe my solution process was a bit overdone and it would have worked without renaming the .bk file but it worked for me now. If the .bk1 file is not needed than I can not explain why the game did not find the saves in the first place. since they were there just the .bk1 file was missing.