I’m having this issue too. I’ve used the browser version for a year and never had this problem (even used it last night). I installed the app to see if it would work but I still can’t sign in. My Epic ID is “Aluminum Rank”.
Edit: It seems to be working now. It was either a glitch or something on my end. I linked my account through this website & the app oulled my account up after reopening it.