It’s unclear if these messages are an expected result of a rather naive/noisy sketch polling the radio and saying it received nothing when it would not yet be expected to, or if they indicate at least occasional synchronization errors in the question/answer exchange of the serial communication between the processor running your sketch and the LoRaWAN module. Figuring that out would be one step.
Nothing seems to reach the gateway. Do I perhaps have to change something at the gateway?
Right now its not exactly clear that your node is transmitting at all, and if it is, if it is doing so on the correct frequencies for your area.
You’ve also said nothing about what your gateway is, or its configuration. You could have a gateway for the wrong frequency band, you could have a gateway not correctly registered on TTN, or you could have one of the notorious rip-off boxes from Dragino (and others) such as the LG-01 or LG-02 that is actually not a LoRaWAN “gateway” at all, but rather another node radio doing a very poor job of impersonating just a fraction of the capability of a gateway - which statistically more often than not would not including the channel/datarate that the node is currently using to transmit.