Oh boy. You are pretty thoroughly hosed. Not impossibly hosed but your system is in dire straights. Messing up sudoers on a no-longinable-root account system is probably second only to
rm -rf / and tied with
sudo chmod -R a-x / in single commands that can almost completely disable a system.
In the future and all you future readers of this thread:
I speak from experience.
You need to boot into some other OS, mount the file system from your server, and re-edit sudoers to make it valid. I have no idea what could be wrong with it. It might just be the file permissions (it should be read/write for root only and no permissions for everyone else).
Since you are running openhabian, you might be able to do this easily if you have a Linux machine or VM that you can mount the SD card to. You can then use
visudo -f /path/to/sdcard/sudoers/file. If you have a spare card and USB SD card reader, you might be able to put a stock raspbian on a new SD card and mount the old SD card from the USB reader.
If not, you need to decide if it is going to be more work to set up an environment where you can mount the SD card’s file system or just rebuild the OS from scratch. Of course, if you have files you need off of that SD card, you will have to do this anyway if you don’t have recent backups.
I had an Ubuntu server that I messed up sudoers on and had to boot into a live CD, chroot, and then edit using visudo in order to get my system back. Hopefully, you will have an easier time of it.
BUT, you are on the right track in solving your original problem.