Connectivity Issues
From what I've observed, it looks like once someone begins writing a new card, anyone trying to view the board get's "Internal Server Error" until the other user has completed their card.
This was on a private board that I've now made public. I looked through your issue reports and didn't see anyone else that encountered this issue. Based on that, I'm wondering if the majority of people are using public boards and this issue is specific to private boards. I tested this theory by having people edit cards on our public board and that's working just fine. So it does appear to be specific to private boards.
Didn't mean to close the issue, my bad!