What is: Fatal error: Uncaught CurlException: 52: GnuTLS recv error (-12): A TLS fatal alert has been received. – in Facebook PHP SDK-Collection of common programming errors

I’m developing a Facebook canvas application and sometimes (3 times in one week) I get this error (to be honest, this time it’s a user getting the error):

Fatal error: Uncaught CurlException: 52: GnuTLS recv error (-12): A TLS fatal alert has been received. thrown in /myURL/facebook-php-sdk-master/src/base_facebook.php on line 1031 

myURL is the URL of my application’s original address (not the apps.facebook.com one).

I can’t find the meaning of this error, the only thing I could guess is that it’s something about encryptions, hence the TLS.

One possible thing (I’m guessing again) is that the application is running on an HTTPS subdomain, but our main domain is normal HTTP. The console is showing an error every time the app runs:

Refused to connect to 'http://www.someURL.hu/' because it violates the following Content Security Policy directive: "connect-src https://*.facebook.com http://*.facebook.com https://*.fbcdn.net http://*.fbcdn.net *.facebook.net *.spotilocal.com:* https://*.akamaihd.net ws://*.facebook.com:* http://*.akamaihd.net https://fb.scanandcleanlocal.com:* *.atlassolutions.com https://*.internet.org http://*.internet.org http://attachment.fbsbx.com https://attachment.fbsbx.com".

But this was like this the whole time and Chrome is showing a warning on the HTTPS icon, saying something about 128-bit encryption and TLS (