Some users report email links not working

Within the last three weeks, we've been receiving reports of users clicking a link from one of our emails and the page not loading. I was in touch with support who said it had to do with Constant Contact's link tracking and the users' email client security settings. But in the past 13 years of using constant contact this has never been an issue, and it is happening across all email clients from Gmail to Yahoo to Hotmail to business email addresses. Is anyone else experiencing this issue? And if so, what have you done to fix it.