You're running into a known issue that was fixed in VMAN 6.0.1 HF1. 6.0.1 HF2 is currently available and is a cumulative release that includes HF1, so I would recommend getting HF2 from your Customer Portal and applying it to your VMAN 6.0.1 appliance. If that still does not resolve the problem, let me know.
P.S. - I've posted the Readme for HF1 here for reference. Your issue is FB270598 in the notes below:
SolarWinds Virtualization Manager Hotfix #1 (SolarWinds-VMAN-v6.0.1-HF-1)
Only install this hotfix on a Virtualization Manager 6.0.1 appliance.
The hotfix provides these fixes:
1. capacity planning calculation fix for datastore free space (FB270598)
2. Hyper-V sample collection unit corrections (FB280780, FB280790)
3. memory leak and other fixes (FB282894, FB281503)
Installing the hotfix
Installing the hotfix replaces the VMan web service for both the server
and any federated collectors. The original files are renamed and saved
for rollback to the original. The install script will restart the Tomcat
service and Virtualization Manager will be running the hotfix.
Follow the steps in the next sections to install or uninstall the hotfix.
1. Use SCP to copy the enclosed VMAN-v6.0.1-HF-1.tgz file to the
admin user's home directory on the master appliance.
2. Log in to the appliance as an admin user using SSH (for example using putty)
$ cd /home/admin
$ tar xf VMAN-v6.0.1-HF-1.tgz
$ sudo bash ./install-v6.0.1-HF-1.sh
1. Log in to the appliance as an admin user, and sudo bash.
$ cd /home/admin
$ sudo bash ./uninstall-v6.0.1-HF-1.sh
After updating to HF2 I still have the problem that one out of 4 clusters is not shown.
I think I should open a case and test your support. ;-)