Download nwnexplorer and try opening the file with it (use the Open icon).
First open the file with BackupMod extension, then after renaming it to mod.
With BackupMod you should see just binary content of the file’s header (in hex, make a screenshot of it). With mod you should either see file contents or nwnexplorer will crash.
You can post the screenshot so we can see what’s going on in there.
Sorry to hear that, but if you haven’t yet, you may consider writing a shell script that will automate making of timestamped backup copies of your module. Apart from catastrophic failure protection, it creates a timeline of recent saves you can easily rollback to if you make changes you don’t like.
I found this in the Vault (warning - untested). It’s for NWN2 on Windows, but the principle should be the same for NWN1: copy module file, compress it (save space), put a timestamp in its name and store it somewhere outside game’s directory tree. You could even wrap it (if you make your own script or modify this one) around the toolset, so the module is backed up every time you close it (aka paranoia mode).
There is also NIT by @Surazal, which seems to have some backup utilities, but I cannot tell if you can do a one-click backup of a module with it.
NIT would not be suitable for this task. The backup it performs are for its own database files and not the Mods.
If you are using Windows 10, you could consider using the Backup using File History and make it only backup your Mod files including the Toolset’s temp folder. You can specify a frequency and then use the File History to restore your Mod.