Error 503: Cannot deploy app to GAE

If you can't deploy an app to a Python or Go instance and getting the following error:
Application: vel-loxal; version: a
Host: appengine.google.com

Starting update of app: vel-loxal, version: a
Scanning files on local disk.
Cloning 35 static files.
Cloning 86 application files.
Compilation starting.
Error 503: --- begin server output ---

Try Again (503)
An unexpected failure has occurred. Please try again.
--- end server output ---
Rolling back the update.
Error 503: --- begin server output ---

--- end server output ---
...it's maybe because you're uploading a static-only app (maybe a GWT frontend?), an app without any executable files. So you should disable appcfg.py's pre-compilation:
appcfg.py update --no_precompilation target/artifacts/GWT-only/