I think I have found a solution. The base for my action was this issue.
It seems that NC25 added check in OC_Util for some config parameters.
This is what I have modified. It would be good for NC maintainers/developers to take a look.
My instance is working correctly now as 25.0.1. However, without knowing why this check was added - it is difficult to answer which function now may be impacted. But on the other hand - this check is preventing my instance from upgrading totally as I have never defined “secret”.