Hello @FirstNameL188902 ,
Are they able to provide the exact error message they're receiving? Just based on your initial description, it sounds like it may be caused by an ad/pop-up blocker, VPN, browser extension, or some other network/security program erroneously blocking the click-track rewrite.
If this is the situation, it would result in an error page when affected contacts try to go to your link in the email, but not when you send them the original URL the rewrite is trying to redirect them to. The only real way to resolve this would be to safelist our domains in whatever program is causing the issue, and would be typically done on a case-by-case basis.
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