if your oscommerce has a "ultimate seo url" then
your seo url setting must be

admin -> configuration -> SEO URLs -> Output W3C valid URLs (parameter string)? "false"

this is important for your members
because url of the activation code do not work with & code.

Here is a sample for activation code link:

before setting:
account_activate.php?activation_id=820f1af38e8a52a d2cc79239be4becd7&activation_code=g2rxyhn03cts

after setting:
account_activate.php?activation_id=820f1af38e8a52a d2cc79239be4becd7&activation_code=g2rxyhn03cts

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