When trying to send email message via Open-Xchange application, error message appears: Error: File upload failed. Processing of multipart/form-data request failed...
ResolutionCheck permissions and ownership of directory /tmp/.OX on Open-Xchange server.
That directory and it's subdirs must be writable for the open-xchange process. The owner of the process is open-xchange:open-xchange. Fix directory owner: chown -R open-xchange: /tmp/.OX/