Which version of Project Server are you using? Since you claim that you can drag and drop on browser, I will assume 2013.
If Excel client is not functioning properly, it could be an issue with the system, RAM. Have you tried doing this with another PC? I consitently observed that my PC with 4GB RAM sometimes cannot handle Excel 2013, PowerPivot and PowerView without stalling..
Prasanna Adavi,PMP,MCTS,MCITP,MCT http://thinkepm.blogspot.com