Plugin dosen’t work with clean WP installation on Siteground

Hello!
3 days no success with plugin. The site hosted on SiteGround, in .htaccess added:
Header set Access-Control-Allow-Origin: *
Any caching option is disabled on hoster’s side.
When I press any fastspring’s button in editor I see just grayed out pop-up form, but can’t to select anything. When I click at form it disappear.
I tried to use Siteground’s WordPress installer and manual installation of WordPress — the results are the same.
Furthermore, with FastSpring Plugin enabled, if to click “View details” near the Plugin’s description there is also grayed-out pop-up window. If to deactivate FastSpring Plugin — everything comes back to normal.
What can I do?

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