I just upgraded to a Note 10+ and kept running into this aggravating issue for days. I tried changing my password and even disabling two-factor authentication without success.
For anyone still having this issue, I stumbled across a Google Play Store review that said the username field is case sensitive even though the app doesn't test for it. As a result, the app thinks it has both logged you in and denied you entry (Schrödinger's cat of user sessions?). If I had to guess, most people are entering their emails similar to
Firstname.Lastname@example.com (I originally did this for each attempt). I managed to successfully log in by entering my email as
firstname.lastname@example.com.
Since I'm not sure if LastPass stores all emails as lowercase, I'll add this step if anyone's still having issues. Check your email format by logging in through lastpass.com (the website) and open Account Settings. You should see it next to Account Email. You can just copy it and paste it into the LastPass app.
tl;dr - LastPass's Android developers forgot a very basic programming step. The login screen's email field is case sensitive. Either it doesn't A) convert a user's email to all lowercase or B) ensure the entered email case matches the email in their system.
Try entering your email as all lowercase. If that doesn't work, copy your email from the Account Settings after logging in through LastPass.com in a web browser. -Scott