I solved it.
I leave the fix here in case anybody else stumble upon thisin the future. It's from the Team17 web but this forum doesn't allow me to post links.
1. Find your backup
The 'backup-saves' folder location:
C:\Users\%username%\AppData\LocalLow\Black Salt Games\Dredge\backup-saves
The file names dredge-save0.bin.bak_0, dredge-save0.bin.bak_1, etc. would contain backups for slot 1, or files named dredge-save1.bin.bak_0, dredge-save1.bin.bak_1 and similar would contain backups for slot 2 etc.
2. Copy the backup file from the 'backup-save' folder to the 'saves' folder
Now copy (Ctrl+C) the backup file inside the 'backup-save' folder and paste it (Ctrl+V) into the 'saves' folder.
The 'saves' folder location:
C:\Users\%username%\AppData\LocalLow\Black Salt Games\Dredge\saves
3. Rename the backup file
So that the game can recognise the file as a correct save, its name will need to be changed. The correct names of the saves in each of the 4 slots (whether they are empty or not) are:
dredge-save0.bin
dredge-save1.bin
dredge-save2.bin
dredge-save3.bin
That means you just need to delete the last 6 characters from the name of the file you copied from the 'backup-save' folder (.bak_0) to create a proper save file. But before you do that, check if the 'saves' folder already contains the save file you want to create. If it does, make sure to move that file outside the folder (if the file is empty or damaged, you can simply delete it) to avoid file conflicts when renaming the backup file.
