Which link do you post in the connection manager?
Should be like:
https://<mycompany>.sop.ondemand.com/sap/sop/addin/excel/
We had issues at the beginning when we used the short version only
https://<mycompany>.sop.ondemand.com
and the files in folder
C:\Users\<your user ID\AppData\Local\EPMOfficeClient\
had not been generated correctly
(cannot remember properly, but both EPMXLClientPreference.XML and SOPXLClientPreference.XML need to be there
If it works not for you but for a colleague, try if you can copy & paste his files here, but only if they dont exist yet and the long link was unsuccessful as well)
If that does not solve and you find a different solution , please update here for me to learn
Thanks
Irmi