Hello again,
I did a few more tests on my past problem, and I figured out I did upload every file in the correct place. I tried renaming the file "itrader.php?", to see if it would use the appropriate 404, and it did.
(see screenshot:

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And when I keep the file name as "itrader.php" in the root*/forums/, it gives me the 500 error.
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Is there any advice you can give me to make this work? I've tried quite a few things, but none of them worked.
Thanks in advance,
WC