Any customer using widgets, even Partner Websites, should switch to a simple button and link to the appropriate RunSignup site directly.
Unfortunately, Apple has released a new version of Safari that prevents session tracking. This is a necessary feature on complex web applications such as a registration.
Note that this issue only comes up in a minority of cases. They have to be using a new version of Safari, they have to have some set of limits on their cookies, and they must not have visited the RunSignup website previously as described in the quote above.
Since originally reporting this issue, we have made some changes to try to work around the issue of an infinite loop of not being able to register. We now show a redirect to a page on RunSignup where those people seeing the issue:
As we discussed a year and a half ago, widgets are not a good long term strategy for putting functionality on a website. This is an example of websites taking more and more precautions to minimize security risks, which is actually healthy for the web.