If you got this error:

PHP Notice: in file /includes/utf/utf_normalizer.php on line 174
Failed opening './../includes/utf/data/utf_canonical_decomp.php'
PHP Warning:  Cannot modify header information - headers already sent by *

It can mean that some file in includes/utf/data or includes/utf is missing. i backed up this directory and then downloaded corresponding original PHPBB files (same version as i have installed - version is visible in admin area)

Extracted files and added into above mentioned directory and it worked, error is away