Mobile issue: Log In fails

I use the Samsung Internet browser ver. 5.4.02.3, with Adguard Content Blocker enabled. (Disabling it hasn’t helped yet.) It’s almost certainly some quirk in the way the cookies are being handled, though no other saved passwords sites I visit have been affected. But my phone is so old and cranky and loaded with data, who knows. It doesn’t look like anyone else is having this issue.