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Using TreeBuilder to put data into XML tree.
Convert that tree to a string.
Parse that string.
XML parser error.
I expect XML library to be able to parse its own output.
Reference example: https://github.com/FinchPowers/python_xml_builder_bug/blob/d98b2422d9ecadbee37e2896a5098abf82b1d7a4/python_xml_builder_bug.py 

Note that you cannot copy paste the code and expect to reproduce the issue because it contains special characters that cannot be copy pasted via a web browser.
Here is the output when run.
<?xml version='1.0' encoding='utf-8'?>
<top><line>2015年03月12日 09:44:54.560 script runner plugin Hello, world</line></top>
Traceback (most recent call last):
 File "python_xml_builder_bug.py", line 17, in <module>
 ElementTree.fromstring(io.getvalue())
 File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/etree/ElementTree.py", line 1300, in XML
 parser.feed(text)
 File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/etree/ElementTree.py", line 1642, in feed
 self._raiseerror(v)
 File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/etree/ElementTree.py", line 1506, in _raiseerror
 raise err
xml.etree.ElementTree.ParseError: not well-formed (invalid token): line 2, column 35

Use \x escapes to construct your example, then.

Updated file where copy/pasting works.

This is a duplicate of Issue5166. Note that in your example, you are not serializing valid XML 1.0: \x1b (ESC) is not a valid XML character, it has to be escaped and, as the discussion in Issue5166 points out, etree does not automatically do that for you.