Hi Karl,
Did you add the roundcube domain of your email provider as described here (https://mailvelope.com/en/faq#not_on_list):
QuoteLoad the website you want to add to the list of authorized domains. Select the Mailvelope icon in the upper right corner of your browser to open the main menu. Select "Authorize this domain". A Mailvelope dialogue to add the new domain will open.
For me the website is https://roundcube.[my-email-provider].org, not roundcube.net. I guess in your case it should be something like roundcube.us1.providedns.com:[port #]
Step 5. of this page (https://mailvelope.com/en/help/anywhere) gives more details and shows a screenshot of the configuration window.
I had the same issue as you until I found this.
Note that in my case the error message still shows in the Roundcube Encryption settings after doing that (and enabling the checkbox), but encrypting and decrypting do work.
Cheers,
Pierre