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I don't know of any port specific issues in any URLs generated unless a 3rd pary module has a problem.
Are you redirecting a normal port 80 at the firewall or port 81 in and out? Either should work if Apache and XOOPS are configured right.
If the user's browser URL has the same base as in mainfile.php it should be fine no matter what port. But they should match.
And, what are you using to redirect?
More info might help.
no max no bling. goes double for mik. triple for the insane aliases of said lunatics
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Not around here anymore. Way too many idiots, so sad.