A user clicked on the update button when this allert showed up.
The updater says it is removing old and then shows the install new window. After clicking this, there are errors that the file can not be written to /programs/nextcloud.
I found out that the original Nextcloud app is not removed. The cause was that an other user was logged in on the same machine.
Logging out that user did not work anymore.
I solved it by ending the client process through the task manager when loged in as the other user. Then I logged that user out. In my own login I manualy deleted all files in /programs/nextcloud.
After this I could install the new 3.0.2 client.
I do not know if this behavior is ‘by design’, but the installer should check for ‘used by an other user’ in my opinion. This is the same check as when one tries to shut down the machine while an other user is still active.