Error "All cacheable requests must have creation dates"

Hi,
I recently installed the Simple Calendar plugin, but I’m having troubles to make it work.
I double-checked the documentation and I’m pretty sure Google Calendar and the API key are correctly configured.
Still, I’m getting this error:

While trying to retrieve events, Google returned an error:

All cacheable requests must have creation dates.

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.

This is the System Report:

### WordPress Installation ###

Site Name: Dominio dei Felini
Home URL: http://dominiodeifelini.altervista.org
Site URL: http://dominiodeifelini.altervista.org
Version: 4.4.1
Locale: it_IT
Timezone: Europe/Rome
Multisite: No
Permalinks: /%postname%/
WP Memory Limit: 64 MB
WP Debug Mode: No
Script Debug: No

### Active Theme ###

Theme: Customizr – 3.4.19
Child Theme: No
Parent Theme: –

### Active Plugins ###

Yoast SEO: 3.0.7
Simple Calendar: 3.0.14
BuddyPress: 2.4.3
NextGEN Gallery by Photocrati: 2.1.23
Ultimate WP Query Search Filter: 1.0.10
SI CAPTCHA Anti-Spam: 2.7.7.7
WPFront User Role Editor: 2.12.4
Enhanced Media Library: 2.1.7
rtMedia for WordPress, BuddyPress and bbPress: 3.10.3

### Server Environment ###

Web Server: Apache
PHP Version: 5.4.40
MySQL Version: 5.1.71-community-log
Server Timezone: UTC
Display Errors: Yes
Safe Mode: No
Memory Limit: 256M
Upload Max Filesize: 20M
Post Max Size: 20M
Max Execution Time: 30
Max Input Vars: 5000
fsockopen: Yes
cURL: Yes
SOAP: No
SUHOSIN: No
WP Remote POST: Yes
WP Remote GET: No (403)

### Client Information ###

IP Address: 176.9.149.79
Browser: Chrome 48.0.2564.116 (Windows)

There’s only one line that worries me:
WP Remote GET: No (403)
However, the error seems to be referred to another problem.
Any ideas?

Thanks in advance for the support.

https://ww.wp.xz.cn/plugins/google-calendar-events/