When Upgrading the Omada-Firmware to V5.15.8.12, everyone should look at configured VPN Profiles, first. 

Using a Hyphen ("-") in the Name is no longer supported after (at least) V5.15.8.12, so existing VPN Profiles using that 
character then (apparently) fail to load and establish a connection after an successful upgrade.

 

(Becomes unhandy, when upgrading a remote side that is inaccessible without an established VPN Connection) 

Ran into this issue on one side, renaming the profiles prior to upgrading on other sides avoided inaccessibility after the upgrade.
 

best,

dognose