Well the plot thickens.
So when my original problem occurred I reformatted the HD and reloaded a brand new instance of TM. Fine. It worked for 2 days, then same error.
So I bought a brand new HD, new cable (Lacie Mobile drive that’s optimized for Mac), once more reloaded the TM. It ran without fault for several weeks. Until yesterday. The same error, despite First Aid running without faults:
So it’s pretty much certain that my external HDs are not at fault and that Mac OS Catalina is making my life harder. Is it possible that the TM codebase can corrupt, or something? I’m out of ideas, and would be very grateful in case anyone has any further thoughts on how the OS installation itself could be at fault.