Wow, this issue is over a year old!
And it’s fixed!
The issue was that treadmill activities were imported into CityStrides incorrectly. The latitude/longitude coordinates were set to 1,1 or 0,0 or 1,0 or 0,1. This made it impossible for some browsers to render. Sorry, browsers!
First, I fixed it so that the issue couldn’t occur for new activities.
Then, I went back and cleaned up all the old activities with these bad markers.
Everyone’s LifeMap should work properly now. 
I think there is still an ongoing issue with people who run too much. 
Some people have so many activities, that the browser just can’t handle displaying all that data. 
The fix for this is being worked on now - you can read about it in Beta: LifeMap performance and you can use it today if you’re a subscriber. If you’d like to discuss this new beta, please do so over in that thread. 