Indeed, and therefore I'm not using the virtual ASG as a time source in my network. But it's not good when times in reports and logfiles aren't accurate.

How can the VM's clock be held in sync with the host's clock? As I already wrote, in my last installation I didn't have that problem at all

Hmm, I just remember an article about that problem I read a long time ago:
HELP: VMware - Fixing Time Keeping Problems with the Guest OS : VMblog.com - Virtualization Technology News and Information for Everyone

I'll try these settings

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Running Astaro ASG virtual appliance / Home power user 100 IP license
Intel S3210SHLC / Intel Xeon X3320 / 8192 MB RAM / 3Ware 9650SE 8-Port / APC Smart-UPS 750XL