I don't use the Roku Mobile App all that often anymore so do not know know if it recently underwent an update, but it was crashing on me last week every time I attempted to open it. I cleared the cache of the App on my Android and now loads/works correctly.
Just a thought.
For Android, go to Settings/Apps. Then select the Roku Mobile App. Select Storage. Then Clear Cache. I also selected Clear Data, so can't say if clearing cache is sufficient.
Normal procedure is to restart phone, but I was able to simply launch the Mobile App again successfully. This was on an Android Moto Zplay, Android v.7.1.1
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102