We administer a small business network (3 Dell servers and about 20 Windows workstations). The installation is mostly Microsoft based Active Directory, Windows Servers, WSUS, etc.
All machines and PCs have current VMware 6.5 and Microsoft updates.
Two days ago the vCenter Server Appliance 6.5 crashed and we needed to reboot all the physical servers to get the installation running again.
We have created two testing VMs with Windows 10 Ent. 1809 for an external company. They work on the implementation of business software which has unclear problems with a Microsoft SQL Server synchronization process in our network. This company is given access to both Windows installations on the VMs via VPN tunnels. They (should) have no administration access to any part of the VMware infrastructure outside of Windows.
As we found out they have removed the VMware tools from one of those Windows workstations which might have rendered this special Windows installation irreparable and crashed the vCenter Server.
Is this a known scenario that improper removal of VMware tools on a client can crash a vCenter Server or should we look for other causes? There are no clues in the logs.
No, this is not a known scenario. The VMware tools are not really necessary to run a VM, they only make it smoother (and allow some features to work in the first place).
The removal of the VMware tools should not affect the vCenter appliance in any way. Additionally, it should not be necessary to restart all hosts just to get vCenter running again. You didn't provide any information about the "crash", so it's next to impossible to come to conclusions about the cause.
You have a support contract with VMware, I'd suggest you create a log bundle and open a case there.