OK. I haven't found a solution yet and I haven't found a way to get information from LabVIEW about the reason for not showing the constructors.
Instead I have created a new assembly (.NET dll) with only the classes needed from my LabVIEW code.
To be sure I can distribute it along with an LLB-file, I am registering the assembly in the GAC.
This is not the desired solution, and maybe I can use the "real" assembly some time in the future, when it can be registered in the GAC. - all its dependencies must also be in the GAC .
But with my current solution, it is very easy to use the assembly in LabVIEW, and all changes are immediately available after closing and re-opening LabVIEW.
Thanks for the attention.