Photos on Catalina will only export recent images Macbook Air, new in the last six weeks, with an existing library brought in via the Migration Assistant. Photos added to the library directly to the new machine export properly. Photos older than the new machine, however, won't export at all. We usually get an Unknown Error (-1) or Unknown Error (-4) for these photos.

iCloud is not used/enabled. We have tried the "Repair Library" process, and it makes no difference.
Suggested by Apple support as a workaround: drag photos directly from the Photos window to a Finder folder.