XF 1.5 Importing phpbb3 to XF 1.5 charset issue

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we're mid test import from phpbb 3.0.14 legacy forums to a fresh install of XF 1.5.16a anyone know why this would be occurring:

An exception occurred: htmlspecialchars(): charset `���6C' not supported, assuming utf-8 in
 /nas/content/staging/[domainname]/community/library/XenForo/Template/Abstract.php on line 520

is this the new database charset or is it likely to be the source database that it's coming from?

any ideas?