I too am testing XOOPS 2.2.4 on a localhost WAMP5 server.
PHP 5.1.1
Apache 1.3.33
MySQL 5.0.16-nt

When in the Extended Profiles Administration page under the "Fields" tab I get the following similar PHP error:

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Notice [PHP]: unserialize() [function.unserialize]: Error at offset 0 of 20 bytes in file c:\wamp\www\xoops2.2.4\kernel\object.php line 344

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I am not sure I know how to do the manual update of the database to fix the problem like you did. Do I need to make the exact same manual change that you did or might my problem be slightly different? Any help would be greatly appreciated!

Here is the point in the code where the error occurs:

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case XOBJ_DTYPE_ARRAY:
if (!is_array($ret)) {
if ($ret != "") {
$ret = unserialize($ret); <-----line 344
}
$ret = is_array($ret)?$ret:array();
}