Error: (403) The referrer null does not match the referrer restrictions…

Hi, I’m using your plugin on my site (listed below in your plugin’s system report). The calendar I’m using is from a Google Apps account and has correct permissions for sharing/public reading.

When I have a properly-formed referrer in Google’s Calendar API Console for this API key, I get the following error:

While trying to retrieve events, Google returned an error:

Error calling GET https://www.googleapis.com/calendar/v3/calendars/REDACTED.calendar.google.com/events?singleEvents=true&maxResults=2500&timeMin=2016-06-19T00%3A00%3A00%2B00%3A00&timeMax=2017-07-17T23%3A59%3A59%2B00%3A00&key=REDACTED: (403) The referrer null does not match the referrer restrictions configured on your API key. Please use the API Console to update your key restrictions.

Please ensure that both your Google Calendar ID and API Key are valid and that the Google Calendar you want to display is public.

Only you can see this notice.

However, if I temporarily remove the referrer from the Google API Console whitelist (which, of course, isn’t safe for a public site), then the calendar events successfully load. Are you aware of any reason that Google interprets the plugin’s request referrer as null and not the actual site domain? If so, is there something that I can change (either server-side or in the plugin/WP settings) to repair this?

### WordPress Installation ###

Site Name: Redeemer Bible Church
Home URL: https://www.redeemercolorado.com
Site URL: https://www.redeemercolorado.com
Version: 4.5.3
Locale: en_US
Timezone: America/Denver
Multisite: No
Permalinks: /%category%/%postname%/
WP Memory Limit: 128 MB
WP Debug Mode: Yes
Script Debug: No

### Active Theme ###

Theme: RBC Theme - 1.0.0
Child Theme: No
Parent Theme: -

### Active Plugins ###

WP Remote: 2.7.6
Yoast SEO: 3.3.4
Advanced Custom Fields PRO: 5.3.9.2
EWWW Image Optimizer: 2.9.3
Simple Calendar: 3.1.2
Regenerate Thumbnails: 2.2.6
Gravity Forms: 2.0.2
Mailgun: 1.4

### Server Environment ###

Web Server: Apache/2.4.7 (Ubuntu)
PHP Version: 5.5.9-1ubuntu4.17
MySQL Version: 5.5.49-0ubuntu0.14.04.1
Server Timezone: UTC
Display Errors: No
Safe Mode: No
Memory Limit: 256M
Upload Max Filesize: 2M
Post Max Size: 8M
Max Execution Time: 30
Max Input Vars: 1000
fsockopen: Yes
cURL: 7.35.0
OpenSSL/1.0.1f
SOAP: Yes
SUHOSIN: No
WP Remote POST: Yes
WP Remote GET: Yes

### Client Information ###

IP Address: 159.203.213.103
Browser: Chrome 51.0.2704.106 (Apple)

https://wordpress.org/plugins/google-calendar-events/