Windows 7 System Update takes hours to complete

On my MacBook Pro i run OSx 10.9.5 with Parallels 10.3.0. The Mac has 16Gb Ram and a 512GB SSD.
I have a Windows 7 Pro, Windows 8.1 Pro and Windows 10 Pro VM. All the VM's have 8GB RAM and 50-60GB disk space.

My problem is that a while back system update for my Win7 VM started taking hours to complete. I cannot pinpoint when exactly it started, but i already had the problem with Parallels 10.2.x. The strange thing is, that it only happens on the Win7 VM, but not on Win 8.1 nor Win10.

For testing purposes, i deleted the current Win7 VM completely (including deleting all data), then i installed a new Win7 VM (just the plain vanilla OS, nothing else). After starting the VM and trying my first system update i have the same problem, at the moment system update is already running for about 2 hours. No error message at all.

Anybody else has this problem or know what I can check in order to get this working again?

Thanks
Gerd