Hello Mario,
In general, Robert's suggestion is just what you need - the Aliases tree is what you need to keep your tests unchanged when the objects hierarchy changes.
However, based on the names in the second case, I can say that your tested .NET application stopped being Open for TestComplete. That is, for some reason, TestComplete stopped recognizing the application as Open, and this is the root cause of the problem that need to be investigated.
Run your application and switch to the Object Browser panel. Do you see TestComplete's icon to the right of your application's process in the Objects tree?
Expand the process node, make a screen shot, and post it here or
send to Support.
Also, let me know the exact version of TestComplete you are using (Help | About...).
