[knot-dns-users] refresh, failed (no usable master) - possible TCP issue

Hello, I have an issue with a zone where KNOT is slave server. I am not able to transfer a zone: refresh, failed (no usable master). BIND is able to transfer this zone and with host command AXFR works as well. There are more domains on this master and the others are working. The thing is that I can see in Wireshark that the AXFR is started, zone transfer starts and for some reason KNOT after the 1st ACK to AXFR response terminates the TCP connection with RST resulting in AXFR fail. AXFR response is spread over several TCP segments. I can provide traces privately. KNOT 2.6.7-1+0~20180710153240.24+stretch~1.gbpfa6f52 Thanks for help. BR Ales Rygl