There could be various reasons. First of all I'd suggest you to check if your plugin/office/Confluence versions are compatible based on the documentation: https://confluence.atlassian.com/display/DOC/Editing+an+Office+Document+from+Confluence
You may also want to check out the known issues page here: https://confluence.atlassian.com/display/CONFKB/Office+Connector+Limitations+and+Known+Issues
Please take a note on all the linked pages on the second link too.
If you fail to solve the issue by the above, you are always welcome to open a support ticket with us at http://support.atlassian.com where we can assist you based on the information in your logs.
Can you try and see if the user in question has permissions to edit the page the excel file belongs to? Can you try to use the same user from a different PC? Very likely this is caused by antivirus/firewall settings that prevent proper communication between Confluence and the client.
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