Workaround: Move your code into different scripts inside an extra Jenkinsfile Repository (or in your Build-Repository), check those files out, load them into variables and call the code as function. Example Jenkinsfile (Main executed)
def HelperScript = load("path/to/helperscript.groovy")
//Do something that doesn't work because too much to load in initial script
//Important statement for loading the script!!!
As the script is not loaded initially it can compile the main Jenkinsfile.
Hope that works for more people then just me