I have the same issue on a Roku Max connected to a 1080P television. MagellanTV made these suggestions:
First, please go to Settings (at the bottom of the left-side menu) and open Video and Sound. At the bottom of the screen, you will see Advanced Settings.
1. If you see Auto - Adaption. Please click the Adaption button and change the setting to Max Start Bitrate. This will cause your device to start playing the film from the lowest resolution and stop when it reaches the highest resolution your device can play. See if this resolves the HDCP error messages.
If your device supports 4K but gets an HDCP message, you will have two additional settings to adjust. Please follow the steps below:
2. If you see the words Protocol and Ultra HD, we recommend starting with Protocol.
a) Change the setting under Protocol to DASH. Try the films again to see if this resolves the HDCP error.
b) If the error persists, click Protocol and return it to Auto.
3. The next step is to try changing the Ultra HD setting.
a) Change the setting under Ultra HD to Disabled
Try the films again to see if this setting resolves the error.
b) If the HDCP issue continues after these steps, please set each setting to:
Adaptation: Max Bit Rate
Ultra HD: Disabled
Please try these advanced settings and let us know if the HDCP issue is resolved. We appreciate your feedback, and look forward to hearing from you.
I tried these and set my receiver to only report 1080P capabilities and made sure the LG TV was actually 1080P. The latter suggested settings from MagellanTV did not apply to my situation as I don't have a 4k HDR TV. Even with verification of the settings and adjusting the Adaptation as suggested, I still get the #20 error and trying to escape it usually meant pressing the Roku home button, but I found pressing the programmable button labeled "1" would make the error message go away and I could keep watching until a started another film and got another error message. A workaround for sure. For clarity, I don't have the issue on any other Roku apps, so I think it is something MagellanTV needs to fix.