Didn't investigate much but appears to be 100% of the time. The test is disabled on Android for now.
Test encounters and unexpected exception:
proxy_tests:proxy_with_credentials: Unhandled exception: set_fail_handler: 3: Underlying Transport Error
Test encounters and unexpected exception:
proxy_tests:proxy_with_credentials: Unhandled exception: set_fail_handler: 3: Underlying Transport Error