::I found the reason. The installation of the .NET Framework 1.1 has changed a setting in IIS in a virtual directory, the one from the Web Service, to use Medium instead of Low. Switching it back to Low resolved the issue.

Are you talking about the Application Protection setting, on the Home Directory tab?

BTW, I just found this mini-article as well:

http://www.devx.com/SummitDays/Article/15674