I'm the author of WP-United.
This doesn't look like a conflict per se -- more just error reporting being turned on somehow.
If it only shows up with WP-United enabled, it is probably your phpBB installation that is turning on error reporting.
To disable it, check:
- that DEBUG and DEBUG_EXTRA are disabled in your phpBB's config.php
- check that no custom file in phpBB has an error_reporting line added (e.g. some auto-installs of phpBB set an error_reporting line).
- Check there is no error_reporting line in phpBB's common.php -- I've noticed some forums that are updated from old versions don't have this line (that existed in old versions but has moved to a different file in new phpBB versions).
- check that you don't have any custom php.ini in your phpBB install that enables error reporting.
Hope this helps.