You must be living on Hawaiï or so, it's still too cold to switch of the thermostat over here in Nijmegen
First of all, from a LoRa point, node and gateway do not "connect". A node just sends packets and any gateway in range will receive these packets and start decoding. All valid packets received by the gateway will be forwarder to the router and eventually (when using confirmed messages), the network will decide which gateway gets the action to send the ACK message to the node.
This means that your gateway will receive the messages on channels it listens on and checks all these messages. So you can never be sure that the messages you receive are from your own node. The gateway has no knowledge about the encryption key so it is not able to see from which node the message was received.
Unless you have special (network debugging) code in the gateway that is able to decode the messages using the application session key of your node.
Are you sure the messages resulting in CRC fails are from your own node? I normally attach an LED to my development boards in order to see when it starts transmitting.
Do you have any means of checking the radio spectrum? I have a simple SDR device that is able to scan the 868 MHz band so I can see if there is any other signal besides my own.
If you don't, hunt Google (try searching for "RTL SDR 868". Something like this is almost a must when doing your own development, it shows you not only if you are transmitting but also on which channel(s), if channel hopping is used, what kind of SF you are using (a bit of guessing is involved there) and if there are other signals interfering with your signals.
I have seen very strange wide-band flashes across the 867-869 region in my office. Not one of my devices and it pops up now and then showing very short high-power burst once every few seconds for about a minute