App Engine to connect cloud sql with error: ApplicationError: 1007 Invalid connection id

I am using app engine on python to connect the Cloud SQL, and it
worked well for few days. But from yesterday, I got the error for any
database query:
OperationalError: could not connect: ApplicationError: 1007 Invalid
connection id.

In my python codes, I used singleton pattern to maintain one
connection for all my database operations, and I am not sure that
caused the problem.