I performed some packet captures direct from MMDVMHost and it looks as though XLX950 is not returning the Repeater ID after the RPTACK. I've checked the source and it doesn't look like MMDVMHost or DMRGateway are validating the repeater ID but HBLink is, causing it to fail. Interestingly enough, DMRGateway does include the Repeater ID when it ACK's the MMDVMHost login.
mmdvm_bm.pcap (successful BM login, showing RPTACK with Repeater ID)
mmdvm_xlx.pcap (succesfull XLX login, showing RPTACK without Repeater ID)
I guess to be inline with MMDVMHost and DMRGateway, HBlink shouldn't validate the repeater ID when it receives an RTPACK from a master, but should include the ID when it sends a RPTACK to a master.
I hope that helps,