CSFB from 4G to 2G MO calls: class 6-Protocol error

Dear experts,
I have some issues in CSFB from 4G to 2G MO calls.
I m getting blocked calls.
Reason shows cause class 6-Protocol error.
What may be the problem?
There is no blocking in GSM cells.
Vendor is Nokia.

Verify TAC/LAC mapping at both core and access.

Have you check that this issue is with some specific UE?

It’s ok (Both are same).

No, it’s not for an specific UE.

Did you check the neighbors are proper created?
I mean the external declarations with the right values.

Yes, Neighbour definitions are correct.

I guess the switch to CSFB to 2G is ON, isn’t it?