Colliding Greek usernames during conversion

Hello,

I'm converting a Greek phpBB2 forum into phpBB3. The board has Greek as the default language and the the problem is that all usernames that use Greek characters only in CAPITAL letters (for example: ΠΑΝΑΓΙΩΤΗΣ) appear as "Colliding usernames and I get the attached message with strange characters. The problem does not appear to usernames that use a mix of CAPITAL and lowercase Greek letters (for example: ΠΑΝΑΓιΩΤΗΣ) but only to those with only CAPITAL Greek letters...

I tried almost anything I read around the forum like to match database's encoding with the file /language/lang_greek/lang_main.php and vice versa, to change database's collation but nothing worked.

After all I deleted the colliding usernames and the conversion run fine at first glance without any strange characters inside posts and usernames. Of course this was not a solution but a test.

Any help will be appreciated!