In version 5.3 when you edit the config file via the admin section it will say that $config[activethreads] is unknown to the base installation, even though it is. To fix this edit your admin/editconfig.php script. Line 36 has a long list of variable names and it ends like this:
-activethreads";
Change that line, to end like this:
-activethreads-";
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Scream
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