SIM7000G MQTT CSSLCFG="convert",1, Operation not allowed

Hello Guys,

I want to establish a MQTT connection to an Azure IoT-Hub with a client Certificate and a client Key.

The certificates are present on the module for sure because I can read them out:

However when I want to configure the certificates to be used by MQTT, I get the following error:

---> AT+CSSLCFG="convert",2,"rootCa.crt"
<--- OK
---> AT+CSSLCFG="convert",1,"clientcert.crt","clientkey.crt"
<--- +CME ERROR: operation not allowed //this response
---> AT+SMSSL=1,"rootCa.crt","clientcert.crt"
<--- OK
I don't know what to do, since the same certificates do work over Wifi.

I hope someone could help me out.

Thanks in advance

Florian