I noticed only by chance that a right-click on the vmdk-file contains the option "Map first volume as Z: (read only)" and indeed this allows to access the VM's C: content. However, with this selection the VM could no longer be opened (started).
I actually would not require access to all of C: My intention is to backup a data folder on the VM's C from the host because it becomes more and more difficult to obtain online backup clients and anti-virus programs which are compatible with the old, but still required guest OS.
Is there a possibility to achieve this, and - if yes - how?