I got a new version 1.0.6 pushed on May 17th, which apparently might be needed if TTN updates the server certificate some day after July 2019:
With this patch we add the ISRG Root X1 CA to the gateway firmware. After 8 July 2019, many servers that use Let’s Encrypt will start serving a certificate chain which leads to this new root, and therefore it must be trusted by clients such as the gateway.
So, for those not having automatic updates enabled: you’d better do so, I’d assume.