This could be someone clicking the registration web page link in an email, or trying to click either the Confirm or Decline buttons in an invite.
First, as always, make sure you have distributed the correct link if you are sending out the Register Page link. You should always send a test email to yourself before sending any email to people via Seatrobot. Click the links yourself in the test email to make sure they are correct.
If the links / buttons are good, there are usually only three possible reasons for links 'not to work' when a recipient clicks them:
Problem #1 - Internet Connection: they’re actually not connected properly to the internet. This sounds unlikely, but it can happen. A recipient could be reading an email they downloaded previously and have subsequently lost connection to the Internet without knowing it.
Fix: check with them to make sure they have an internet connection. Ask them to load your company website for example, or download their latest emails.
Problem #2 - Security or firewall issue: their email system / computer network at their place of work does not allow them to launch web browsers from their email, or their web browsers are configured to not be able to go to 'outside' web pages. Sometimes company security systems remove parts of the link when displaying it in their employees' emails. This is quite common in companies such as banks, legal offices, financial institutions, for example.
Fix: ask them to right-click and copy the link or the button and paste it into the address bar of a different web browser. Alternatively, they can try to access using their own mobile phone. As a last resort, ask them to forward the invite to their personal email, and click the link on their phone or even using their computer at home.
Problem #3 - Internet Explorer users: some companies still use a deprecated version of Internet Explorer. This affects less than 2% of Internet users in the US. If this is their company's default web browser, it may launch automatically when the recipient clicks the link in the email from you. You may therefore find that a number of people from one particular company have the same issue.
Unfortunately Seatrobot (and Microsoft!) no longer support these old browsers, which present severe security issues and cannot handle the latest web technologies.
Fix: Again, ask them to right-click and copy the link and paste it into the address bar of ANY other browser - e.g. Google Chrome, Safari, Firefox, Microsoft Edge etc.