Adding vCenter or a ESXi host when the vSphere version is 6.7 update 2 (6.7.0.30000) to the latest version of the plugin for VMware (12.2) fails with no error.
The debugging traces shows the following:
1 VMWVIM :00280 508 16726 140115.902513 SOAP RetrieveServiceContent Server 'https://vcenterserver:443/sdk' Header 'urn:vim25/6.7.2'
0 VMWVIM :00280 29 0 140115.910143 Soap error is (null)
1 VMWVIM :00280 5 0 140115.910155 Login success=FALSE
0 VCENTER :00280 19 0 140115.910160 Failed login: '(null)'
3 PLGLIB :00280 380 0 140115.911158 Send error reply: '(null)'