Just had a vCenter 6.7 Update 3i, where I did not see the VUM plug-in.
there was no information in the logs, or any other places, the only thing was that en vSphere-UI Client toke a long time to start, and it toke a long time to login to it.
I could see that there was some plug-ins that had problems with loading, this was an old Veeam and Nimble plug-ins.
After unregistering the Veeam and Nimble plug-ins, and restarting the vSphere-ui ad vSphere-Client, I cloud use the VUM.
Note sure if it was the Veeam plug-in, Nimble plug-in og the combination of them that coursed the problem.
The Nimble plug-in will be installed again, when it updated. The customer do not use the Veeam Plug-in, so that will not be installed again.
I will update the blog, if I get/finds more information.