ErrorException: Push notification failure: cURL error 28

[hdrcweb]

Hi
I've been getting these on daily basis in my server log, this example is related to a post made where lots of images were successfully attached but it varies from post to post.
Any ideas why this is happeneing?
Thanks

#0 src/XF.php(219): XF\Error->logError('Push notificati...', false)
#1 src/XF/Service/PushNotification.php(267): XF::logError('Push notificati...')
#2 src/XF/Service/PushNotification.php(179): XF\Service\PushNotification->handleResults(Object(Generator))
#3...

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ErrorException: Push notification failure: cURL error 28

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