After installing the latest wsus-updates I could not edit Maintenance Plans on my workstation any longer.

The software was not responding and I found out that the history of the reports in SSMS was gone. It seems to be a problem with a .Net4 update. Uninstalling KB2840628 and rebooting my machine fixed it.

There is a connect item (new SqlConnection("context connection=true") will throw TypeInitializationException->System.Security.SecurityException) for it, other people are facing connection problems after this update as well.

So if you run into this problem, please vote for it and try the workaround in the meantime.