Hello Kaspar,
Since the error occurs only after you run the VI a second time, it is likely that there are some references that weren't properly closed before the VI finishes execution. Please ensure that you unregister any events using the Unregister For Event VI (in the Functions Palette->Connectivity->.NET->Unregister For Event) before the VI finishes execution. Please also ensure that any related object references (connected as inputs to the Register Event Callback VI) are properly closed using the Close Reference VI (Functions Palette->Programming->Application Control->Close Reference) before the VI finishes execution.
To make sure that you find all of the Register Event Callback functions in your application, you can use LabVIEW's Find and Replace tool to find any instances of a given function.
To use the Find and Replace tool go to Edit->Find & Replace. Then select the object that you would like to find in your application. You can do this by clicking on the box next to "Select Object", selecting Functions, and then choosing the function you want to search for (for Register Event Callback, select Connectivity->.NET->Register Event Callback). Click Find. All instances of the function being searched for in your application should be listed. You can double-click an item in the list and it will show you where that specific instance is located.
I hope that helps!
Brock
Applications Engineering
National Instruments
