Thank you very much, KMatle and Erel, for your support!,
I also suspected that there is not a bug but a misunderstanding - but the signs indicated that there must be a bug after all.
This was the source of the Bug:
In Main, the project has a CustomBuildAction with a path to a no longer available Java.exe.
Therefore, B4A reported that Java exe cannot be found and I overlooked the Build-Log note that it is processing the "custom action", which is more precisely the custom build action.