I haven't has this before, but you might want to try going to the setup tab and try readding the VC server. Do you have VC added by hostname, IP or localhost? Once you re add it, try to just click on the inventory button to see if SMVI fetches the vCenter Inventory OK.
This is caused by an issue with the VMware vCenter DB not properly updating during an upgrade, and being unable to provide a valid virtual machine listing when SMVI is querying it for a list of the virtual machines which are contained in each datastore.
This issue has been found to be caused by the vCenter database. Reinstalling vCenter will not resolve this issue.
Creating a new database and then reinstalling vCenter has been found to resolve this problem.
Note: If you are unable to create a new database for vCenter, please contact VMware technical support directly to troubleshoot the problem with the vCenter database