I am new and had the same issue. I have an iPhone 8 plus with the latest OS and an iPad Pro latest OS. The problem occurred on both.
I use a password manager and I allowed it to create my Roku account password and it logs into the web site fine. I tried using the password manager to log into the App and also tried to type it in manually. I removed the APP and put it back. Everything failed.
Here is what worked for me. I changed my password using the website to a very simple password. ( Stupid idea I know ) but it worked.
I'm going to guess the App has issues with very complex passwords.
Thanks, Rob, that’s the deal. There’s no need for any reinstalls. The app doesn’t accept any special characters for a password, while the web site does. So the solution is to change your password so that it only contains letters and numbers.
Quick update to share. We've released a new version of the iOS app that should help address this issue. Please make sure to update the Roku mobile app and let us know if you're still running into this behavior.