Just today this issue has arisen with me as well. I recently updated to 8.035 from 8.010 and there has been a lot of strangeness, and now this.
Insert fx, when clicked, do not appear anywhere. I can somehow tell they are appearing “behind” something, but there is nothing to be “behind”, as no other windows are open in Cubase other than the arrange window and the channel editor. No other applications are open. The only way to make a plugin’s interface appear after trying to open it is to go to the Windows taskbar and click on the Cubase icon. There were no changes of any kind between last night and this morning when the problem appeared. I have restarted Cubase several times with no improvement.
Update: It’s affecting all windows, all VSTi’s and fx, even the channel editor window doesn’t appear. It’s window madness! It is impossible to work on anything like this.