Q2686 - SMS that can't be read

I’ve noticed that quite often on some SIM-cards, AT+CPMS? indicate that there is an SMS on it that doesn’t show up if the SIM is used in a cellphone or used with Q24-modules.
AT+CMGL=“ALL” lists no SMS at all.
AT+CMGR=a, b=index of the invisible SMS, a!=b shows OK but a==b gives a CMS ERROR that indicates invalid index.
Once the index has been found, AT+CMGD=b works to get rid of it… AT+CMGR=b now shows OK and no invisible messages are there any more… until the next restart…

Any ideas?

Tested on
Q2686, FW6.63