Well, the problem with these notifications is that UBB.threads is not "Mobile Friendly" by Google's Standards
. For a page to be considered "Mobile Friendly" it must render completely legible at a maximum page width of about 400px (a small mobile device's screen).
When you have the Portal Box sidebars on, that's 550px alone, without even the content in the center of the page. With that center area you're adding about 300px more to the equation (as you add more to the width, displaying content on a smaller device means that it must be zoomed out to get everything on the page, which gives you the exact errors you're seeing). Errors generated from this would be:
Page appears not mobile-friendly
Text too small to read
Mobile viewport not set
Links too close together
We're working towards a Responsive Design
for the front end of UBB.threads in future revisions using a bare minimum smartphone screen as the standard (not tablets).
You can take and validate Isaac's Sandbox
to see how things will be working as they're added to the stock package. Where a mobile UBB.threads is "getting there", it's still not ready to be released into the wild at this time.