well, eventually it turned out to be system wide problem. While I was doing apt-get update of my debian it took exceptionally long to "Read package list", and finally it crashed with well known "no space left on device" . I checked fs with badblocks and btrfscheck, but they didnt report anything wrong. I also checked syslog and kern.log too. And actually.. after rebooting everything seems fine (scratchbox too) but we'll see for how long..

edit:

the problem remain unsolved until i found this:
https://btrfs.wiki.kernel.org/index...._lots_of_space. so the solution was to type btrfs fi balance couple of times. yay