Chantal Cerboni 

It sounds as though something happened to your package.  Did this error appear when you tried to open the package (the *.sdlppx file) the first time?  Or did it happen after the project was created and you tried to open the project to work on it?  There are a few KB articles available on this topic, perhaps try and check them to see if one fits your scenario?

Chantal Cerboni 

It sounds as though something happened to your package.  Did this error appear when you tried to open the package (the *.sdlppx file) the first time?  Or did it happen after the project was created and you tried to open the project to work on it?  There are a few KB articles available on this topic, perhaps try and check them to see if one fits your scenario?

Hii Paul ,
thank you for your prompt reply. I have finally identified the issue: on my new PC, they had installed Norton AV by default, without telling me. And I had installed McAfee, which is the AV I have always used. So I have uninstalled Norton, and everything works now. But I have not precisely found out if the issue was caused by Norton or by a potential conflict between both AV programs...

Thanks for coming back and confirming Chantal Cerboni .  That's a useful bit of information that might help someone else in the future.