I restored a time-machine backup onto my new SSD. Since then, I can't build my Xcode project anymore. It gives me an error at the #import line, saying 'Three20/Three20.h' file not found.
How is that possible, the files are still there, in the same place. I don't even know where to start searching. Any ideas or clues?
your header search files probably were cached inside Xcode cache folder and when you restore from time machine, these cache files weren't restored.
if you want, try to compare the header search folder in both projects to see the difference.
The project should have these: (assuming your project is located on the same folder as three20 is)
HEADER_SEARCH_PATHS = $(BUILT_PRODUCTS_DIR)/../three20
$(BUILT_PRODUCTS_DIR)/../../three20
../three20/Build/Products/three20