As mentioned, you might want to check your atlassian-confluence.log which is located in <Conf_HOME_Dir>/logs/ in order to further investigate this issue.
If you are running on MySQL server database, most likely the cause of the issue is due Max Allowed Packet configuration which is discussed on our knowledge base article here:
Hope it helps.
I'm Gliffy Technical Support. Could you please open a ticket at http://support.gliffy.com? I'd like to ask you for a copy of your current log file.
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