Call drop cause 98

Hi All,

A few cells in our network has many call drops with cause 98.
I can not find any information what this means.
The only information that I have about this is:

Cause value = 98 Message type not compatible with protocol state
This cause indicates that the equipment sending this cause has received a message not
compatible with the protocol state (section 8.4).

Does anyone knows what this means?

Thanks in advance,