trac.util.datefmt - strftime format must be str
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strftime() argument 1 must be str, not unicode
which is provoked right there in trac/util/datefmt.py - the reason is that the argument to format_datetime is not checked in any way, and if it is unicode then python 2.4 will reject it. The fix is easy: just wrap "format" in "str()".
(I assume that the JsGantt read the format from a trac.config item passing it through without looking at it. I haven't tested that though)