ok, thanks for the hint. mystery resolved: my AP and STA were both on WDS mode, looks like batman is having issues with 4-address mac frame format. Taking them to standard mode resolved the issue.
But then Im having many tcp out-of-order, dup-ack issues, making streaming almost impossible. Not sure if these are related to MTU stuff. Will need to spend time on this. If any threads regarding this may help a lot.
Best regards