We had some trouble updating a managed solution within crm. The hint after solutionimport was:
A managed solution cannot overwrite the LocalizedLabel component with Id=<some guid> which has an unmanaged base instance. The most likely scenario for this error is that an unmanaged solution has installed a new unmanaged LocalizedLabel component on the target system, and now a managed solution from the same publisher is trying to install that same LocalizedLabel component as managed. This will cause an invalid layering of solutions on the target system and is not allowed
It turned out, that there were unpublished changes in the system (especially changes which referred to an attribute, which was changed in another solution than the managed)
publishing all customizations prior to import solved this problem.
Lesson learned: try to publish all customizations befor importing solution(s).