By default, it seems dev_appserver Task Queue handling does not work well behind a reverse proxy, like ngrok. This patch allows requests that don't match a host to still be processed.
Specifically, commenting out the raising of the ModuleDoesNotExistError and instead returning None for the _module and inst variables so the task can still be processed.
This is around line 816 in _resolve_target() in dispatcher.py in the 1.9.86 gcloud app Python Extensions:
\platform\google_appengine\tools\devappserver2\dispatcher.py