Same thing started happening to me today. Both retail and classic WOTLK are stuck in the update/scan loop and I get the same error as you when I try to launch WoW directly via WoW.exe
UPDATE: I reinstalled both retail and WOTLK classic, deleted the entire /WoW original folder that was still left after the uninstalls and now a newly installed WoW is working normally again. So give it a try if you have the same issue.
(Before this I basically tried all the possible blizz techical support fixes (deleting caches, connection troubleshooting etc.) and none of it worked)