FWIW I had this issue yesterday on my Mac. I could connect but I got these same weird one letter tables. Looked exactly like what @taraas shows above.
My situation ended up being conflicting installs of unixODBC and some interplay between the database access packages (DBI, and odbc). I had to remove one of my conflicting unixODBC installs (Homebrew in my case). Which left me with a MacPorts install of unixODBC that seemed to be working. Then I had to remove DBI, and odbc form R.. and iir I blew up tidyverse too, although may not have needed to. Then I restarted my R session and rebuilt DBI and odbc packages. I was then, after a reboot, finally able to get stable access to my sqlserver.
I suspect, but cannot prove, that I had built different database access packages under differing configurations of unixODBC.
you mileage will vary...
I'm looking forward to when R Studio just ships with its own unixODBC stack.