You've been invited to collaborate on a timeline but, when clicking the Open Timeline button in the invitation email, you can't access the shared visual. You don't get any error message - you just get directed to the application's homepage and you can't find the timeline in the Shared with me folder.
You may run into this issue if you are on MacOS 10.14.4 (18E226) and you're using Safari 12.1 (14607.1.40.1.4).
Normally, when you click on Open Timeline from inside the collaboration invite email, the system communicates to Office Timeline Online that you've accepted the invitation so we can give you access to the shared timeline. An issue in Safari 12.1 breaks this communication, preventing our tool from knowing you are collaborator on the timeline - which is why it won't give you access to it.
This issue will be solved as soon as Safari releases a fix for it. In the meantime, to accept the invitation and get access to your timeline, simply right-click on the Open Timeline button to copy the link behind it, and then manually paste the URL in your browser.