I've tried that several times. I've removed the app, unplugged the roku for 5 minutes, re-installed the app -- no change, there's no opportunity to login, it just crashes.
There is no login for this app - it is set up to look at your cable modem to confirm that you are an actual customer and then take you straight in. I have found this to be a bit flaky in execution.
I've seen this off and on - a call to Spectrum fixed the issue last time. I don't know specifically what the tech did, but he put me on hold for a few minutes and then asked me to try again and it worked.