Lion Server 10.7.3 server has a confirmed bug where if Windows users mount SMB shares, they are unable to open PowerPoint files directly off the server. This problem doesn’t affect Word or Excel files, and doesn’t occur when Macs try to open the same PowerPoint files directly off the SMB share.
When this occurs, Windows users will either get the error “PowerPoint was unable to open or save this document. Please ensure that you have access privileges to read or write the document and that it is not encrypted” (Windows XP w/Office 2003) or “The path or file name […] is invalid. Please check that the path and file name are correct” (Windows 7 w/Office 2010).
There is no workaround to this issue currently, other than copying the PowerPoint files down locally to open them, then copy them back to the server if modified.