The upgrader produces this warning several times:
[phpBB Debug] PHP Warning: in file [ROOT]/upgrader.php on line 154: Illegal string offset 'limit'
The problem is that the check on line 152 no longer works on PHP 5.4:
The limit element does not exist, butreturns true instead of false. Attempting to access that element throws a warning and returns the first letter of `$orig_column_type . ':'`.