FYI, in case this helps somebody else:
http://webgrabplus.com/documentation/installation/linux states that
Step 2 : Install mono. Currently mono-complete has too be installed to (reason currently unknown)
This is actually not necessary. The reason the app doesn't work with mono on it's own is because two dependencies are missing (they are shown if it's run with MONO_LOG_LEVEL=debug set)
apt install mono libmono-system-core4.0-cil libmono-system-web4.0-cil
was enough in my case (Ubuntu 16.04)