Text encoding problem with old files not newly created ones

MBA running 10.13.6

I keep a lot of plaintext Notes in a shared DropBox folder, synced between an iMac (also 10.13.6) and my MBA.

The programs I use to access those notes (The Archive and nvALT) suddenly stopped displaying the notes. nvALT gave a Stopped Responding message in Force Quit. The Archive did not.

I tried opening the notes using other tools, including BBEdit and TextEdit. BBEdit showed a row of upside down question marks. TextEdit showed nothing. But I was still able to open and read those notes on the iMac.

On the MBA, I created a new note in BBEdit and saved it to the shared folder.

Now both The Archive and nvALT were able to display the note content.

So, newly created notes display fine, but old ones do not, on the MBA.

I have no idea how to proceed on the MBA, and would be willing to try anything. I am asking here, rather than from the various app developers, because this seems to be a system-wide problem on my MBA.