This is interesting - I took your LabView project, which you created to test creating a Network Variable from the Calibration Constant custom control I posted earlier, and after observing that the Network Variable could be created, I pursued a divide and conquer until I understood what was happening. First, I must point out that what I posted in the .zip file earlier was scrubbed a little before posting - I changed the names of some of the labels and re-initialized default values. Anyhow, working from within the project you posted I replaced the scrubbed .ctl files with the originals and again I was unable to create the Network Variable. I then replaced the .ctl files one at a time until I found the one causing the problem. It came down to 'calibration constants - solenoid fule.ctl' file. Cutting to the chase, all I needed to do to make this work was to change the name of one of the labels of one of the cluster items making up that custom control. But, it's more than that. I would then change the label name back to what it was and I'm still able to successfully create the Network Variable. I demonstrated this repeatedly for myself just to be sure and got the same results everytime - can not create the Network Variable using the original .ctl files, change a label name and then back again and the problem goes away(???).
