My guess, it is related to the new DVSwitch. I also am running both YSF Reflector and XLX reflector on the same server. XLX would not run as DVSwitch was opening some of the ports for itself that XLX needed. I uninstalled DVSwitch which still allows the use of MMDVM_Bridge to bridge both YSFReflector and XLXReflector to a master hub, in my case hblink3. The buster repository though is still installed on the server. I have been running 3 different YSFReflectors in the past without that error ever occurring, all coexisting with the older stretch version of DVSwitch. I think it is related to DVSwitch as I have also been having problems with P25 and NXDN Reflectors coexisting with the new DVSwitch buster version. One time I did see that same double free detected error when my P25 Reflector shutdown.
I would like to go back to the stretch version of DVSwitch but that link is no longer available. I was hoping Steve would bring back that link so we could test to see if the error message occurs now with the older DVSwitch stretch version. If so, then we would feel better about posting to G4KLX concerning this problem.