is your system bootable?
if not you might need to do an upgrade with a full install dvd (usb)
it’s quite possible that you wore running out of space seeing how btrfs does copy on write and you wore upgrading your whole OS you might have ran out of space
I don’t use btrfs and even if I did I don’t know how to turn off copy on write I think there is an option in yast->snapper to turn off snapshots
if you can’t boot the system get an iso of 15 write it on disk and do an upgrade from disk (which might also fail because your root is out of space)
you should get the 15 live disk mount your existing root and clean some snapshots http://download.opensuse.org/distribution/leap/15.0/live/
FWIW I also had the same error message about running the Ruby debugger after a message about the partition not being bootable. It was right at the end of a massive copying of files and was the last thing I wanted to see. However I removed the DVD and rebooted and it started up fine into Leap 15. I’m no expert here but I had the feeling that the upgrade was confused about what type of boot was required. I am not using UEFI which might have been a factor.
Calling the YaST module 'inst_update_partition has failed.
More information can be found near the end of the '/var/log/YaST2/y2log' file.
This is worth reporting a bug at http://bugzilla.suse.com/.
Please, attach also all YaST logs stored in the '/var/log/YaST2/' directory.
See http://en.opensuse.org/Bugs/YaST for more information about YaST logs.
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