Maybe I'm the only person this happened to or everyone else already knows about it. I'm a noob. But in case not:
When you click on the orange plus sign to enter a race category, the page goes grey and you might see a message that the content cannot be opened in a frame.
The fix is to be logged in at zwift.com in another tab first, it's the communication between zwiftpower.com and zwift.com that is trying to open and failing authentication.