I'll defer to Nibbler on the technical side of this, rather than swallow the foot I had already put in my mouth!
He is correct, of course, that the relative path should be "board/".
However, I tried a test install of cpg1.4.3 with phpBB2.0.19 in a subdirectory, the way you have it installed, and I was not able to duplicate your problem. Bridging went smoothly and worked correctly on 4 separate tries. For what it may be worth, I'll outline the steps I took, and the results in the db.
One note - you listed udb_base.inc.php - v 1.3 in your first post. Did you mean revision 1.13? The most current revision on CVS is 1.14 (posted just a few hours ago).
Files: phpbb2018.inc.php revision 1.11 and udb_base.inc.php revision 1.13
Admin Tools >> Bridge Manager >> Start bridging wizard
On the first screen of the bridging wizard, "phpBB version 2.0.18 or better" is checked.
On the next screen, for forum URL:
http://localhost/gallery/board. For the relative path to phpBB config: board/. For cookie prefix, the default: phpbb2mysql. "Hide unused form fields," checked.
On the next screen, use post based groups: no.
When the bridging wizard finished, I then hit logout, and got the "you are not logged in" error, which is nothing to worry about. I then clicked on login in, and was redirected to the phpBB login page, with the URL:
http://localhost/gallery/board/login.php?redirect=../../gallery/. After logging in, I was logged in on both the board and gallery.
In the database, after bridging, in the cpg143_config table, enable_bridge was 1.
The cpg143_bridge was exactly like yours except for the different URL:
short_name phpbb2018
license_number
db_database_name
db_hostname
db_username
db_password
full_forum_url http://localhost/gallery/board
relative_path_of_forum_from_webroot
relative_path_to_config_file board/
logout_flag
use_post_based_groups 0
cookie_prefix phpbb2mysql
table_prefix
user_table
session_table
group_table
group_relation_table
group_mapping_table
use_standard_groups 1
validating_group
guest_group
member_group
admin_group
banned_group
global_moderators_group
recovery_logon_failures 0
recovery_logon_timestamp
The differences between my test install and yours, going by what you have posted so far, are that: 1) your installs are on a subdomain, 2) I'm not sure we are using the same version of udb_base.inc.php, and 3) you have cpg_redir.php in yours.
The info above from a working bridge should give you a basis for comparison with your install. If you still can't get it working, then I'll be glad to dig further. I can't set up an install on a subdomain right now, because my server isn't configured for subs, but I can change that, and try a test install in a subdomain later tonight (my time: GMT-5) if that will help.