Your screenshot shows that your client connected to the server's WIBU, so I'm not sure why you are not able to obtain the license. The 'no slot free' sounds like someone else is already using the license.
Perhaps a firewall setting? Maybe try temporarily turning off the Mac firewall under Preferences > Security on both machines?
I was able to get this to work with my WIBU under Leopard (10.5) with my Mac serving my Windows laptop, but was not able to get it to work under Snow Leopard (10.6) a year or so ago ...
Hopefully someone else has tried this and knows what might be causing the problem.
Cheers,
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB