@MarkusL Hi there! So you are using Spark Dev also. If I am not wrong about this. App code was invalid could be cause if there are other files in the very same folder as your .ino file.
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So you might have to create a new empty folder and place your .ino file inside the Spark Dev is quite picky on this. Below is another thread that people, including me, faced the same problems.
Spark Dev: app code was invalid - #15 by Leonardo - Cloud - Particle -
If you have libraries that you need to use that are only available in the Web IDE. Open those files you need and do a copy and paste on to a blank notepad and save it as it's respective type. (Examples like .h or .cpp For your case
flash-eeprom.h) Then put this files in the same folder as your .ino file.
Then in your .ino file, just simply put a #include "flash-eeprom.h"and it should be working.
Hope this helps