Conversion from excel to zoho during upload does not work

Conversion from excel to zoho during upload does not work

when I try to upload an excel 2010 xlsx file, the conversion into zoho sheet format does not work, and consequently I cannot view the uploaded file in the preview.

when I repeat the upload now with a much simple excel 2010 file (less functions etc) it works

Q: are  there any known features in excel that prevent zoho to perform the conversion ?

Thanks,

Achim