phpbb_prepare_message() uses $user->lang['POST_ID'] which is not defined in language/en/install.php (or any other file there) giving errors like this in conversion from 2.x:

: 189077 Message parser error:

(where there should be some text like "Post ID" before the first colon)

A quick scan through functions_phpbb20.php shows some other possible offenders:

• COLLIDING_CLEAN_USERNAME
• COLLIDING_USER
• COLLIDING_USERNAMES_FOUND