Not sure I know what you mean by
I've deleted phpBB forum's table prefixes and then works
Based on your issues that you mentioned:
1) Connector did not know the phpbb table prefix
2) can not find encode_ip or session_begin
What the issue is:
1) The connector is not able to load the phpbb common.php file (which loads the config.php and other files from phpbb)
The most likely cause for these issues is the "forum directory" not being set correctly in the plugin parameters for the connector. If this forum directory is set wrong then it will not be able to load the phpbb files and will cause these errors...
Default for the "forum directory" is "forum" which means that your website is at
www.yoursite.com
and your forum is at
www.yoursite.com/forum
If they are not installed in the same website and database then the connector won't work... I assume that since you got the database to not give an error that they are indeed in the same database...
Greg