In changeset fe6be0426e0d the format() method was changed. Unfortunately it does not catch all unicode decode errors. 

I think line 482 of logging/__init__.py should be modified:
to this (add 'replace'):

s = s + record.exc_text.decode(sys.getfilesystemencoding(), 'replace')

http://hg.python.org/cpython/file/f35514dfadf8/Lib/logging/__init__.py#l482


Here is the stacktrace we get:
{{{
Traceback (most recent call last):
  File "/usr/lib64/python2.7/logging/__init__.py", line 838, in emit
    msg = self.format(record)
  File "/usr/lib64/python2.7/logging/__init__.py", line 715, in format
    return fmt.format(record)
  File "/home/modbau_esg_p/djangotools/utils/logutils.py", line 32, in format
    msg=logging.Formatter.format(self, record)
  File "/usr/lib64/python2.7/logging/__init__.py", line 482, in format
    s = s + record.exc_text.decode(sys.getfilesystemencoding())
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 662: ordinal not in range(128)
Logged from file base.py, line 209
}}}