Recently I experienced problems with VMware Update Manager. When working with the virtual infrastructure the VI client suddenly displays a message stating that my session with VMware Update Manager was no longer valid and that the plugin would be unloaded. At first I suspected that network issues were the cause of the problem as at this client site we have separate vCenter and Update Manager servers for security reasons. So I created a affinity rule keeping both servers together to rule out network issues between the two servers. Unfortunately the error messages kept popping up. I had no idea what to do and was thinking about accepting this issue but this goes against my nature and pride.
Then, last Friday I stumbled upon a VMware KB article describing my problem exactly.
The problem described in the article was exactly the problem I had. Bottom line? The VMware Update Manager server was equipped with McAfee antivirus software which was scanning the Update Manager Repository. This caused the session with vCenter and the VI client to fail.
The solution for this problem is as simple as it is logical. We exclude the Update Manager repository from on-access scanning and the randomly disconnecting sessions are gone.