I recently experienced a bug in Spark with the send file feature. I was out of the office, but Spark was still logged in on my desktop. A coworker was testing the send file feature, tried to send me a file, but since I was not around to accept it, the “Accept/Reject File” dialog box appeared with no response on my end. The co-worker gave up and signed off, however, the box to accept the transfer did not go away. When I got into work, I hit accept, but since the user that initiated the transfer was no longer signed in, it caused Spark to lock up and freeze. I had to terminate the task in task manager and restart Spark in order to get it working again. I hope this is enough information to replicate the problem. Thanks for your help.
This was a known bug found in 1.0.3 which has since been fixed in the 1.0.4 release due out at the end of this week.