Updating 3.2.0 to 3.2.1 throws Error 500 and twig lexer.php error. Breaks upgrade

Update per instructed in the phpbb board admin pages. Downloaded the update package, unzipped on local computer. Uploaded /install and /vendor to phpBB server and pointed browser to the install directory.

www.somesite.com is currently unable to handle this request.
HTTP ERROR 500
 
From the logs:
NOTICE: PHP message: PHP Recoverable fatal error:  Object of class Twig_Source could not be converted to string in /usr/local/www/somesite.com/forum/phpbb/template/twig/lexer.php on line 147

This is reported lots of times in the forums, but a reason/fix is not given afaik.