We found that the issue was related to access control between two QAP temp folders for two accounts, one of them requiring admin right. It is hard to tell what caused this situation but it seems that launched in a certain way, QAP was trying to create its temp folder in a folder where it was not having the write access. This seems to be a very specific situation.
Using "%TEMP%" as QAP temporary folder (in "Options", "General" tab) solved the issue. Using this folder is safer because it is not restricted to any user access right (as far as I know). And it is now the default folder for new QAP installations.
Jean