These errors (especially user 2's) may not be related to disconnects. But i would check user1's Preferences > Privacy to see if there is some problematic settings. Maybe removing all privacy settings will get rid of that error.
Have you tried profiles for these users and make them start with new one and see if this helps? Profile is stored in C:\Users\User\AppData\Roaming\Spark (history is stored in this folder, so do not delete it, just move it).