I changed the proxy settings to use manual proxy instead of system proxy and it works now correctly. Hopefully error message can be improved if the update step can not determine the correct proxy server.

There's some connection problem. The button on toolbar shows cached results.

I get following message, when trying Help > Check for Updates. I connect to internet using proxy and it is set correctly under Preferences > Updates > Connection > Use Proxy server.

Error checking for application updates.

However if I use the Update icon on the toolbar, I get

You have the latest version.