Time Machine preferences - time estimate wrong, progress bar doesn't show any progress When I start a backup (or when it is automatically started) I get some estimation about volume (394,2 MB of 2,58 GB) and backup time (e.g., "5 Minutes"). But even after hours this estimation does not change and the progress bar does not proceed.

Any idea? Is there any way to check what really is going on between OS X and my Time Capsule?
You can open a Terminal and run the command tmutil status or check Console to see exactly what your Time Machine is up to; you can have a look at James Pond's website for help in interpreting them.

Unfortunately, Time Machine doesn't provide very detailed information on what it is doing, but this might give you a clue that gives you something to search for!