Flashing FMU using `make target upload` fails

I always flash my Pixhawk 4 by using make px4_fmu-v5_default upload. There has never been any problem with that. Now, out of nowhere, I suddenly get the following error:

ninja: error: unknown target 'upload'
make: *** [Makefile:219: px4_fmu-v5_default] Error 1

What happened here? What did I do wrong?

OK, deleting the build folder actually and trying again solved the problem. Can anybody shed some light on why this happened?