For three days now, I have only been shown sleep history for the last 5 days. From Connect instead the data are complete. Also on GC it recorded me 2 afternoon naps, while the night naps are displayed on the clock

Have you tried a long power down reset (not a factory reset and you wont loose HRV data)
Try a long reset , if possible also whilst unit is connected to Garmin Express via cable.
Hold in power button until the watch powers off completely. Then wait say 15 -20 s and then restart the watch. There are numerous reports that this type of restart assists in sorting some issues - without loosing our weeks of HRV data!