I come across an interesting third party problem that I wanted to share with you.
One Citrix VM built on WinXP SP3 with IE8 could not open via the internet explorer in the PWA, Project Files with the Project Client. OpeningFiles from the Project Client was working.
The Problem was caused by Citrix XenConverter (this is converting the workload from a Server or Desktop virtual machine to the XenServer.)
This is a Known Limitations in Citrix:
Open Files – XenConvert cannot copy a file in use by another application on Windows XP. To ensure that a conversion includes a file, close that application before converting.
After the XenConverter was disabled everything was working fine