The plugin is already registered on the site (correct GUID), but I cannot generate an offline licence (done using steps from this manual: http://download.milestonesys.com/MIPSDK/MilestoneSolutionPartnerLicensingFramework.pdf).
The problem is that the LRQ does not have any CustomData section for your plugin. We are currently looking into whether this should be a strict requirement or not, but for now can you please set the CustomData property to any other string than null - e.g. string.Empty.
Oddly, it worked - thank you for your help!
I couldn’t find relevant info in the SDK documentation, so I think maybe writing a note there would be helpful for other devs as well.