Problems transferring large files via Zoho Excel plug-in

Problems transferring large files via Zoho Excel plug-in

I have just started using Zoho. It looks like a great service. However I have been unable to transfer my large desktop Excel workbooks to Zoho even though I am logged in and using the Zoho plug-in. The transfer appears to work up to ~6MB but fails on 7MB. Ultimately I want to be able to transfer files in excess of 20MB. I am getting the following error:
'Unable to save. Encoding is not proper.'
Has anyone seen this problem? Any recommended solutions?