Line 44 of phpbb\template\twig\defineparser.php

throw new \Twig_Error_Syntax('Invalid DEFINE', $token->getLine(), $this->parser->getFilename());

incorrectly throws an error when you use define to set a variable to the value of another e.g.

<!-- DEFINE $RANKS_TITLE = RANK_TITLE -->

gives

Fatal error: Uncaught exception 'Twig_Error_Syntax' with message 'Invalid DEFINE in "template.html" at line xx' in phpbb\template\twig\tokenparser\defineparser.php:44

however these all work fine

<!-- DEFINE $RANKS_TITLE = something.RANK_TITLE -->
<!-- DEFINE $RANKS_TITLE = (RANK_TITLE or RANK_NAME) -->

Commenting out this line works around the issue so I think it should be expected to work this way.