After hours of web-searching, reboots, and wrong turns, the answer in my case ended up simply being to un-install and re-install Titanium Backup. When I launched the re-installed version it recognized the .txt key on my SD card (as it had on all the previous failed attempts), showed me as the user of the "Donate" version (as it had on all the previous failed attempts) and let me get to (and update) the Super User version (as it had on all the previous failed attempts). But for some unexplained reason, it finally began to behave properly and become usable. Woo Hoo.
FWIW, I used the now properly functioning TiB to restore my old Google Maps 5.8 which had always been much zippier on Sprint Hero than any 6.x.x version has been.