Sorry, I didn't notice that.
I have no experience with SAMD uarts, and I don't know that GSM modem.
You could search for the string in the library and have a look,
whether that is a meaningful message, or an error.
From common sense,
that message could be generated when an expected message of the modem does not arrive,
so maybe a misconfiguration that changes the emission of packets,
or a problem of the connection to the modem.