@idefix37 That was it! Thank you for the link.
OK so now we know, this wasn't even our Windy: https://community.windy.com/topic/6766/unable-to-sign-in-within-app
Thanks for the video @eric603 , this is one of the reasons why we always ask for screenshots. :) Would you mind keeping the link here as a deterrent example for other misled customers? Thank you!