Package Manager error

Hello
I got a weird problem:

I am working on 3 computer systems all OSX. The current project was "ported" from 2018.3 to 2019.2.4f1. Version change went without problems on my Main System.
Then I copied the project (by usb stick) to my 2nd computer system. I could immediately open it without any problems. Works like a charm.
Now I tried to open THE SAME FOLDER that is on the usb stick on my 3rd system. Unity version is all the same on all 3 systems. BUT om my 3rd system I only get errors:

Unity Package Manager Error...
An error occurred while resolving packages...
nuget.mono-cecil: The package tarball at path ...Users/.../Library/unity/cache/npm/packages.unity.com/nuget.mono-cecil/0.1.5-preview/package.tgz cannot be found.

A re-import of the project...or a manual modification of/users/.../Packages/manifest.json file.

Relaunch or retry does not work. I did not have any of this problems on my 2 other systems....

What can I do to solve this????

 

Nobody?