While waiting for the response from the support team, I'd like to hear from my peers here.
I had an email asset with a link that has mtk_tok enabled; the link worked properly during the test, meaning the link opened. Believing the test result, we sent out the email, and many responded to us "the link isn't working". I realized that the length of the mkt_tok is significantly different (60 more chars, to be precise) so that the page has returned 414 error (URL too long).
Though there seems to be some resolutions on the website server end, I wonder if somebody else noticed this before.