Ok, built a new test site, with default settings, Imsanity plugin installed the first thing and the options page DOES appear.
So there must be some kind of incompatibility with some setting in my other sites.
It’s not only that the menu is invisible, is that I can’t access the url “…/wp-admin/options-general.php?page=imsanity/settings.php” in those sites. It gives me the error:
OK I found it! The incompatibility is with the plugin Developer Mode. The admin user that is promoted to developer is unable to see or access the Imsanity settings.
In order to do that, I must deactivate and delete the plugin. And Imsanity settings reappear.
Could you take a look on this issue? I would love to be able to use both plugins.