Error messages in new WDM install

I’ve just built a fresh install of WDM 5.7.2 (plus hotfix) to replace a server being retired, using a newly-built ESXi VM running Server 2012 R2. The install seemed to go well, but when I try to find the Wyse devices on the local LAN I get blue error messages popping up.

From the Devices tab, then the Find Devices button… If I  use the IP Range method I get “Unable to get server details. Please ensure that installation is successful and run again”. If I try a subnet discovery, I get “Exception occurred discovering devices in specified IP range. Object reference is not set to an instance of an object”.

Both messages have me baffled, I can’t find any useful information on the former, and none at all on the latter, so I am really hoping that somebody here can help. Advice would be very welcome, please!