I was finally able to use my (sponsor) details to log into ECAS... I had been inputting my country of birth as England, since that matches what I put in my application. It wasn't working, so I tried a few things. Finally tried changing my country of birth to "UK - British subject" and it worked! It's weird because my husband's details worked just fine with "England" as the country of birth. Anyway, in case anyone else is having trouble with ECAS, looks like you might just have to try a few different combinations.