vCenter Server Appliance root password change not working

I am using VCSA 6.5.0.11000 Build Number 6816762 (german language).

Today I tried updating it with the 6.5.0.12000 Build-Nummer 7119157 updates (via the management interface https://vcenter.xxxx.xxxx:5480/#/appliance/update?locale=de )  . It did not work.

The error was:

"Appliance (OS) root password is expired or is going to expire soon. Please change the root password before installing an update."

So I went to the Admistration, entered the current password and a new one.

But the password was not changed and the error thrown reads (in german): "Das Kennwort konnte nicht festgelegt werden" which roughly translates to "The password could not be set"

As expiration date for the current password there is listed "Fri Jan 01 1971".

VCSA had been installed on Nov 19 2016 (V6.0.0).

Any thoughts how to solve the problem? Why can I log in in the vcenter, but not change the password?