GAE module: "Request attempted to contact a stopped backend."
Fixed by shutting down all instances (on all modules/versions just to be safe).
I am currently experiencing an issue in my GAE app with sending requests to non-default modules. Every request throws an error in the logs saying:

Request attempted to contact a stopped backend.

When I try to access the module directly through the browser, I get: 

The requested URL / was not found on this server.

I attempted to stop and start the "backend" modules a few times to no avail. I also tried changing the default version for the module to a previous working version, but the requests from my front-end are still hitting the "new", non-default version. When I try to access a previous version of the module through the browser, it does work however.
One final symptom: I am able to upload my non-default modules fine, but cannot upload my default front-end module. The process continually says "Checking if deployment succeeded...Will check again in 60 seconds.", even after rolling back the update.
I Googled the error from the logs and found almost literally nothing. Anyone have any idea what's going on here, or how to fix it?