Thanks for your interest in playing Solar Plexus! :)
[Update: Oh, actually, directly opening this URL might be quickest way to make it work: https://html.itch.zone/html/7210576-697954/index.html ]
A workaround may be to right-click on the section of the page showing the error message and choose to open the frame in a new tab. (Exact menu option will depend on your browser.)
Alternatively, it may be necessary to use a more recent browser version. (I’ve encountered this error previously when using an older version of Firefox.)
As I understand it, the error message is related to a change in how browsers/servers support embedding games in a web page. This is why either “un-embedding” the game or upgrading the browser should hopefully workaround/solve the issue.
Sorry for the delay in reply. Brains. :/