Ever since Openfire v3.2.1, a change in the server settings can cause Spark clients to lose their connection during a whiteboard session (using the V2 Whiteboard plugin). If the user tries to either paste or add an image larger than 1Mb, the Spark client connection drops.
The fix is to add the "xmpp.parser.buffer.size" property to the Openfire server "System Properties". Prior to adding the property, Openfire uses the default value of 1,048,576, which causes connections to be dropped when the packet size being sent is too large. Increase this number to allow for larger images (max property value is 2,147,483,647). See the following for more information about the property and why this was changed in v3.2.1:
Thanks to Ryan Graham for troubleshooting with me, which lead him to suggest this as a fix.