Do you have an example of the endpoint your trying to get to?
My guess would be your trying to access an endpoint that doesn't return anything, the web server still tries to serve you a atom feed but fails. I have had this numerous times while working with SData until I find the correct endpoints for the Resource Kinds I'm wanting to work with.
Couple of tips
SData URL takes the following form;
<protocol>://<hostname>:<port>/sdata/<application>/<contract>/<resourcekind>?<queryparameters>
Couple of useful system urls which may help you discover registered endpoints easier;
<protocol>://<hostname>:<port>/sdata/$system
(List all system endpoints)
<protocol>://<hostname>:<port>/sdata/$system/adapters
(List installed adapters)
<protocol>://<hostname>:<port>/sdata/$system/registry/endpoints
(List of registered endpoints)
You may find help more readily available from the following resources;
http://sdata.sage.com/ (Sage SData Portal)
http://sage.github.io/SData-2.0/pages/core/0100/ (Sage SData Core Specification 1.1)
http://sdata.sage.com/SDataForum/ (Sage SData Forum - Useful Resource) (Forum unfortunately has been discontinued)