Recent changes to the conversion software appear to be buggy. Specifically, changes appear to escape strings in some rows of the phpbb_config table used in conversions as follows:

select * from phpbb_config where config_name like 'convert%';
-----------------{}------------------------------------------------------------------------------------------------------------------------------------------------{}++{}----------

-----------------{}------------------------------------------------------------------------------------------------------------------------------------------------{}++{}----------

-----------------{}------------------------------------------------------------------------------------------------------------------------------------------------{}++{}----------

As a result, the converter fails after the old database connection information is submitted. When unserialized, a NULL is returned instead of an array.

The database should look more like this:

-----------------{}-----------------------------------------------------------------------------------------------------------------------------{}++{}----------
| config_name       | config_value                                                                                                                    | is_dynamic |
-----------------{}-----------------------------------------------------------------------------------------------------------------------------{}++{}----------
| convert_db_server | a:4:{s:4:"dbms";s:22:"phpbb\db\driver\mysqli";s:6:"dbhost";s:9:"127.0.0.1";s:6:"dbport";s:0:"";s:6:"dbname";s:10:"klein-putz";} |          1 |
| convert_db_user   | a:2:{s:6:"dbuser";s:4:"root";s:8:"dbpasswd";s:0:"";}                                                                            |          1 |
| convert_options   | a:2:{s:10:"forum_path";s:16:"../../klein-putz";s:7:"refresh";i:1;}                                                              |          1 |
| convert_progress  | a:3:{s:4:"step";s:38:"current_table=16&skip_rows=2097987";s:12:"table_prefix";s:6:"phpbb_";s:3:"tag";s:7:"phpbb20";}        |          1 |
-----------------{}-----------------------------------------------------------------------------------------------------------------------------{}++{}----------

Yes, phpBB 2 conversions are still happening more than a decade later. I do about half a dozen of these a year for clients.

I'm pretty sure the issue is on line 135 of install/convert/convertor.php:

            $src_db->sql_connect($convert->src_dbhost, $convert->src_dbuser, htmlspecialchars_decode($convert->src_dbpasswd, ENT_COMPAT), $convert->src_dbname, $convert->src_dbport, false, true);

The ENT_COMPAT parameter was added.

This flag was added elsewhere in this program since the phpBB 3.3.0 version. All instances should be tested and fixed if contributing to this problem.