Catalina issue: File I/O, file not found error (-43)

Hi there. I recently upgraded to Catalina (10.15.5), and I now get an error message when I save files to an external drive.

The error message only seems to happen with one software application: MOTU's Digital Performer (music recording software). When I save a project file, I get an error message that says...

Digital Performer has encountered an error.

OS error reported by File I/O:

File not found (-43)

Additional information:

(And then there is no additional information.)

Fortunately, the file is still saved, but I get this error message every time. It only happens with Digital Performer (not with Logic, for example), and only with this hard drive.

The hard drive is a Lacie rugged 1 TB drive that is formatted as exFAT—which is how is came. Another external drive (which does not have this issue) is formatted as Mac OS Extended (Journaled). Does Catalina have problems with exFAT?

Things I've tried to resolve the issue:

—Worked with projects on my internal hard drive: No error. 

—Started a project on the internal drive, then moved it to the external drive: I get the error again.

—Started a new project on the external drive: Same error.

—Done a "save as" on the external drive: Same error.

—Ran disk utility on the external drive: Same error.

—Re-indexed the external drive: Same error. 

—Checked the privacy settings in system preferences. I currently have Digital Performer set to Full Disk Access. I tried removing that access too. Either way, I still get the error message. 

Any suggestions?

I'd like to avoid having to reformat the external drive, since I have about 300 GB used. I can't help thinking it's some kind of permissions issue, but I'm not sure how to change that. Currently everything on the external drive is set to read and write. Is there more to do than that?

Thanks.