@Peacemaker Indeed, change the log from the openhab console. SSH into your RPi and use the openhab-cli console command. You will then be able to use the log commands to set the log level.
To turn back to the default, better use:
log:set DEFAULT org.openhab.binding.nikohomecontrol
OFF will also completely eliminate the ERROR, WARN and INFO messages.
All the extra logging should end up in openhab.log.
It is strange this starts happening with more of your items. There is another post with similar strange behaviour. One of the things being looked into there rogue external access. As you have indicated the port of your RPi is not accessible from the outside, the only thing creating an outside access is your Google Home. A next step could be to try to disable that?
Another suggestion, try to put an INFO log statement in all of your rules. You would be able to see when a rule is triggered, and maybe derive something from that.