Protocol error please verify that the server and tdevice are compatible

Hi,

I have set up a survey with one main table related to many others that have image fields. In the beginning the ODK Services sync was working perfect, but when I recorded many images I received the following error "Protocol error please verify that the server and the device are compatible".

(I m using ODK Sync Endpoint 2.1.2)

What I have to change in ODK Sync?
Thanks!