AT+COPS=? returns "ERROR"

We use the bg96 module in our IOT devices.

Last week we got a returned device from a customer. The device did work a 21 month just fine and then it suddenly stopped connecting.

After investigation, it i did see that the bg96 just does not connect anymore.
E.g.

AT+QGMR
BG96MAR02A08M1G_01.012.01.012
OK

AT+CPIN?
+CPIN: READY
OK

AT+COPS=?
ERROR

The “AT+COPS=?” only takes a few milliseconds.
I have tested the SIM card in another device: it connects just fine.

What could have caused this (to prevent this from happening in other devices)?
And what can be done to fix this?

I don’t have an answer but I also use the BG96, so following this conversation incase I get this issue.