Multiple things may be in play:
- Check to make sure that sv_disable_motd isn't set to 1, which is CS:GO's default value.
- The game's MOTD panel isn't visible after the initial on-join MOTD is displayed and dismissed.
- Popup panels are broken in the Steam Client Beta.
- Panorama lacks support for MOTDs.
If your issue is one of the first two,
VGUI Cache Buster (disclaimer: I wrote dis) handles those cases transparently so developers don't have to finagle their own workarounds for cross-game compatibility (see the various CS:GO-specific plugins for displaying HTML pages).
Neuro Toxin also posted a similar, but different minimal solution (externally hosted page using a query string instead of a location hash) right as I was making this post, should you want to slipstream it into your own plugins and not worry about an external plugin dependency.
For the other two, you'll just have to sit tight and hope Valve fixes them.
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