Openwebif is installed and active by default, so it should work.
Can you ping your STB from your PC?
Can your VU+ perform online update?
I guess that you should cross check twice the IP address of your VU+ to be sure to use the right one.
If it still doesn't work, reflash an image, configure it (wizard) and try to connect to Openwebif this should work without changing anything.
Pr2
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Sat: Hotbird 13.0E, Astra 19.2E, Eutelsat5A 5.0W
VU+ Solo 4K: 2*DVB-S2 + 2*DVB-C/T/T2 (used in DVB-C) & Duo 4K: 2*DVB-S2X + DVB-C (FBC)
AB-Com: PULSe 4K 1*DVB-S2X (+ DVB-C/T/T2)
Edision OS Mio 4K: 1*DVB-S2X + 1*DVB-C/T/T2