Logging in my webapp is set up so that log messages are written to sys.stdout. Writes to sys.stdout are picked up by ErrorCollector defined in master-runner.py and written to file. This fails with UnicodeDecodeError when I try to log unicode strings with funny characters. If ErrorCollector wasn't intercepting writes to sys.stdout, they would work fine, so I assume ErrorCollector should be able to deal with unicode.
A quick fix that worked for me was to replace in master-runner.py this: