How to open guidbedit:
https://support.checkpoint.com/results/sk/sk13009
Then follow steps here do detect what object is locked:
https://support.checkpoint.com/results/sk/sk127232
This will give you a hint in what window DNS still is used.
My guess will be in a blade that used to be enabled and now is disabled. DNS config is still there but blade options are hidden. Workaround then is to enable blade, go to the option and remove dns server and then disable blade again.
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