If the host is unable to ping the BD SVI (which is its gateway), please double check if the host/endpoint has the correct gateway configured.
If BD SVI is pingable from host, then make sure you have permit contract between L3out and host EPG.
Are the EPGs and L3out in same VRF?
Do you have contracts setup for the EPG and the L3out?
From the leaf where you learn the VMs, are you able to ping them?
Under L3out are you allowing the subnets you want into ACI? Alternately you can use 0.0.0.0/0
Have you tried using the pre-upgrade check feature? Does any of the items fail on that?
At this point, please take packet capture via SPAN or monitor capture on the switch to see if it is losing packets.