I am also experiencing the low power message. I purchased a ROKU Express in April. 2019. I've always used the ROKU supplied wallwart and cable without any problem until 2 days ago when the low power message appeared. I've tried disconnecting and reconnecting the power supply without success. When I reconnect the white power indicater light is on and the welcome screen works for a few seconds then the power light starts blinking and the LP message appears. Within the next few seconds the screen goes blank.
A second Roku Express that I had ordered for a TV in another room arrived today. Given that it is the same model I tried the wallwart and cable that came with it. The results were exactly the same.
It appears to me that the Roku device is bad.
In reading the online blogs I learned that software updates are periodically sent to Roku devices. Could a recent update be the source of the problem?
Given the short life of the first device I'm likely to return the second one I received today.
I would appreciate any help you can provide in resolving this matter.