Turns out the DB was corrupted. Not sure what caused it.

In my case, the CLPack Repair Utility did not fix the corruption.

But here's the workaround:

1. Ensure all users are out of Lacerte.
2. Create a full backup of all clients in the affected Tax Type.
   The above creates a full working DB of the Tax Type.
3. Rename the existing Tax Type DB folder, e.g. IDATA to IDATA-current
4. Move the backup folder into the same location and rename it to IDATA.
5. Repeat the process for all affected Tax Types.

Thanks, @JAS & @john_samuel for their assistance.