Thanks for posting this! We're aware of this problem; it's performance-related.
It affects people on certain hardware configurations. Some people might see it 1-in-100 times, others see it every time. On the whole only a relatively small number of people ever see it.
Glad you were able to get through! We're still investigating the problem; it's difficult because I never see this on my own computer, so a few people are giving me info about where it occurs for them.