I have upgraded NSM from 126.96.36.199 to 188.8.131.52. There aren't any issue happens as the known issues list. But there is an issue happen when I upgrade the sensor OS (model: NS7200 failover pair) from 184.108.40.206 to 220.127.116.11. However, after reboot, the sensor still connect to NSM successfully. I don't know what that.
Did the error clear after you rebooted the sensor?
I already have my NS series sensors running on 18.104.22.168 software so hopefully won't run into this issue.
I can't say I've seen that reported by other customers, so if the reboot doesn't solve the issue, gather a sensor trace and open a support ticket.