There is a forum with ~2mln messages.
Source forum - phpBB 2.0.22, MySQL 5.0
Target forum - phpBB 3.0.2, Oracle 10.2

Convertor starts but gives 'MySQl Server has gone away [2006]' error on the moment of getting list of colliding users:

SQL ERROR [ mysql4 ]

MySQL server has gone away [2006]

SQL

SELECT username, user_id, user_posts FROM phpbb_users WHERE user_id IN (<list of 96 user_id's>)

BACKTRACE

FILE: includes/db/mysql.php
LINE: 158
CALL: dbal->sql_error()

FILE: install/convertors/functions_phpbb20.php
LINE: 1897
CALL: dbal_mysql->sql_query()

FILE: install/install_convert.php(1003) : eval()'d code
LINE: 2
CALL: phpbb_check_username_collisions()

FILE: install/install_convert.php
LINE: 1003
CALL: eval()

FILE: install/install_convert.php
LINE: 203
CALL: install_convert->convert_data()

FILE: install/index.php
LINE: 402
CALL: install_convert->main()

FILE: install/index.php
LINE: 274
CALL: module->load()

The same query in phpMyAdmin works fine.

If I manually fix a problem with colliding users removing them from forum DB using phpMyAdmin, I get another error:

MySQL server has gone away [2006]

SQL

SELECT MAX(forum_id) AS max_forum_id

And so on... Looks like convertor starts giving an error for any SQL against source DB. All SQLs which give errors work fine in phpMyAdmin. So, it's not a DB misconfiguration error or hardware problem.

If I change target DB from Oracle to MySQL (the same instance but different DB/user) - convertor works fine and converts my DB without any error.

My topic in Convertor forum: http://www.phpbb.com/community/viewtopic.php?f=65&t=1225875
I can provide any additional info, can test any DB, any software and configuration changes, etc - I have a dedicated server with root control.