One post I read here recently suggested this could be caused by corruption in the Roku's memory. It said this could be cured by restarting the Roku as that would clear the memory and start over fresh. I've not experienced this problem so I don't know if the advice is any good, but restarting can't hurt to try. From the home page: Settings > System > Restart system.
Let us know how you make out.
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T