Hello.
What do you mean – “built-in WP Plugin installer” ? It deactivate admin Plugins page? Or admin PLugins instalation?
Regards,
Alexey.
When you activate your Coming Soon plugin, If you then go to the Add New Plugin in WP Admin, and search for any plugin, then click the details of the plugin you want to install, you then get a pop up with the plugin details, at that point if you try to click the Install plain link/button, you will noticed that the link/button is disabled.
Cannot replicate on three different installs.
Be sure to check that you don’t have any active plugins that may conflict. Also, if using a ”premium” theme, check if the theme is using the WordPress stock Jquery (is quite common for many of them use a CDN hosted Jquery, with other versions than the WordPress core one. Also check if the theme (if in use) using lot’s of scripts that may be included the wrong way.
