TransformError: Error during transformation after upgrade from Plone 5.1.1 to 5.1.4

Plone Support
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I decided to pin lxml and plone.app.textfield to their older versions that ship with Plone 5.1.1
(lxml 4.1.1 and plone.app.textfield 1.2.10)

This is additional traceback information that is showing up on the console, interestingly this mentions a UnicodeEncodeError. (I might not have been paying attention, so these tracebacks may have been there before the pinning.)

Traceback (most recent call last):
 File "/home/ubuntu/buildout-cache/eggs/plone.app.textfield-1.2.10-py2.7.egg/plone/app/textfield/transform.py", line 63, in __call__
    return output.decode(value.encoding)
  File "/usr/local/lib/python2.7/encodings/utf_8.py", line 16, in decode
    return codecs.utf_8_decode(input, errors, True)
UnicodeEncodeError: 'ascii' codec can't encode character u'\u201c' in position 261: ordinal not in range(128)