Some characters in stdout of jobs cause scheduler interface to display "a string literal was not closed." (Exact message depends on browser)

The web interface sends <show_state> commands to the Job Scheduler.
The message appears if the xml returned by the Job Scheduler is corrupt.
In particular, this error happens when a job writes certain characters to stdout. The Job Scheduler sets the text from stdout as the state_text of the job. The state_text appears as an attribute of <job>. Certain characters seem to close the element before the attribute is closed.

When this happens the web interface displays an error message and doesn't work anymore.
When the web interface tries to get an answer from the job scheduler, the job scheduler logs to stdout (and not to scheduler.log):
error : xmlEncodeEntitiesReentrant : input not UTF-8
output conversion failed due to conv error
Bytes: 0xBF 0x5B 0x33 0x67
I/O error : encoder error