Blueprint works in PlayMode but not in Built

Hi,

I have a problem concerning the Built of my project.
Everything works fine in the play mode, but when building for Windows the majority of Blueprints won’t work properly.
The only warnings in the Output Log are a few of these " PackagingResults: Warning: Serialization issue: null value found in templates".

Has anyone an idea how to tackle the problem?
Thanks a lot in advance.