But I cannot uninstall some solution, it results in a error message that some component/attribute is required by the System solution.
Mailchimp is one of those, the message is "This solution cannot be uninstalled because the 'Attribute' with id '1d9fe874-221d-eb11-80c5-00505690c158(activityid)' is required by the 'System' solution."
I think that the activityid is a attribute which should exist in the system solution by itself, so the message doesnt make sense.
When using the "Managed Solution Deletion Tool" of the XrmToolBox, it says "This solution has no dependent solutions and can be deleted immediatly"