Disabling the popup, making it Draft also works – disables the lines this plugin adds to the web page, so my https: site works again.
Just in case it helps, here are all the bad lines added by the plugin – note the https:// links instead of https as used by all other plugins at my site:
<link rel='stylesheet' id='mc4wp-form-basic-css' href='https://www.example.com/wp-content/plugins/mailchimp-for-wp/assets/css/form-basic.min.css?ver=4.2' type='text/css' media='all' />
...
<link rel='stylesheet' id='popup-maker-site-css' href='https://www.example.com/wp-content/uploads/pum/pum-site-styles-1.css?generated=1520893534&ver=1.7.6' type='text/css' media='all' />
...
var pum_vars = {"version":"1.7.6","ajaxurl":"https:\/\/www.example.com\/wp-admin\/admin-ajax.php","restapi":"https:\/\/www.example.com\/wp-json\/pum\/v1","rest_nonce":null,"default_theme":"1680","debug_mode":"","popups":{"pum-2046":{"disable_on_mobile":false,"disable_on_tablet":false,"custom_height_auto":false,"scrollable_content":false,"position_from_trigger":false,"position_fixed":false,"overlay_disabled":false,"stackable":false,"disable_reposition":false,"close_on_overlay_click":false,"close_on_esc_press":true,"close_on_f4_press":false,"theme_id":1684,"size":"medium","responsive_min_width_unit":"px","responsive_max_width_unit":"px","custom_width":"640px","custom_height":"380px","location":"center top","position_top":"100","animation_type":"fade","animation_speed":"350","animation_origin":"center top","overlay_zindex":"1999999998","zindex":"1999999999","close_button_delay":"500","triggers":[{"type":"auto_open","settings":{"delay":"500","cookie_name":["pum-2046"]}}],"cookies":[{"event":"on_popup_open","settings":{"name":"pum-2046","key":"","time":"19 hours","path":1}}],"id":2046,"slug":"rabais"}},"disable_tracking":"","home_url":"https:\/\/www.example.com","message_position":"top"};
var ajaxurl = "https:\/\/www.example.com\/wp-admin\/admin-ajax.php";
var pum_debug_vars = {"debug_mode_enabled":"Popup Maker: Debug Mode Enabled","debug_started_at":"Debug started at:","debug_more_info":"For more information on how to use this information visit http:\/\/docs.wppopupmaker.com\/?utm_medium=js-debug-info&utm_campaign=ContextualHelp&utm_source=browser-console&utm_content=more-info","global_info":"Global Information","localized_vars":"Localized variables","popups_initializing":"Popups Initializing","popups_initialized":"Popups Initialized","single_popup_label":"Popup: #","theme_id":"Theme ID: ","label_method_call":"Method Call:","label_method_args":"Method Arguments:","label_popup_settings":"Settings","label_triggers":"Triggers","label_cookies":"Cookies","label_delay":"Delay:","label_conditions":"Conditions","label_cookie":"Cookie:","label_settings":"Settings:","label_selector":"Selector:","label_mobile_disabled":"Mobile Disabled:","label_tablet_disabled":"Tablet Disabled:","label_event":"Event: %s","triggers":{"click_open":"Click Open","auto_open":"Time Delay \/ Auto Open"},"cookies":{"on_popup_close":"On Popup Close","on_popup_open":"On Popup Open","pum_sub_form_success":"Subscription Form: Successful","pum_sub_form_already_subscribed":"Subscription Form: Already Subscribed","manual":"Manual JavaScript","cf7_form_success":"Contact Form 7 Success"}};
var pum_sub_vars = {"ajaxurl":"https:\/\/www.example.com\/wp-admin\/admin-ajax.php","message_position":"top"};
/* ]]> */
....
<script type='text/javascript' src='https://www.example.com/wp-content/uploads/pum/pum-site-scripts-1.js?defer&generated=1520893534&ver=1.7.6'></script>
