I know it has worked before but I have had lots of issues in the past since the AdobeID password does not seem to work here, blocking my account after a few failed attempts.
This is super frustrating (especially since I have admin rights). How can it be that the single sign-on does not work properly on Target Classic?
Apologies for the inconvenience !!!. To access Adobe target Classic , you need to have Target classic credentials which can be created raising a client care request.
If you are a supported user for your company account you can contact Customer Care. To contact Customer Care by telephone, call 1-800-497-0335. Toll free numbers outside the United States can be found on the Adobe Digital Marketing Customer Care Regional Phone Numbers page (https://helpx.adobe.com/contact/dma-external/DMACustomeCareRegionalPhoneNumbers.html). When asked to select an option for your product, press 3 to contact the Target team.
As of now Decommissioning of Test&Target (Target Classic Workflow) - no access to UI is planned in NOVEMBER 2017 and no Adobe ID SSO integration is in the pipeline for future Target Classic releases .
We would recommend you to transition to Adobe Target / Standard. Please see the following link for more help on Transition.