The logs confirm that the Libre Office command timeout is being exceeded. Can you run the following command on your data node(s) to increase the timeout to 120s
sudo -u zimbra zmlocalconfig -e oo_command_timeout=120000
sudo -u zimbra zmlocalconfig -l
The final command forces a reload without you needing to restart the server to pick up the change.
I am hoping that 120s should be enough for Libre to convert the ppt, but you may have to increase the timeout to 180s if 120s does not work.