I suggest that you connect to the serial console and look if there is still an IP address configured on the vlan "mgmt".
Is the management port "up"?
What does "show vlan mgmt" tells you?
In case the mgmt port stays down, I suspect that you run into an issue described in the following Knowledge article:
https://gtacknowledge.extremenetworks.com/articles/Solution/MGMT-port-stays-down-after-upgrade
An upgrade to any version higher then 15.3.5.1will solve this permanently.
Workaround:
Enable Auto Negotiation setting on the peer switch as interim solution.