Thank you Charles, Michael for your input.

In the original design CS1 was directly connected with Vcc.
When I put CS1 low the RTC keeps the correct time/date.
I used a jumper to do this. When I reset the system,
the reset routine hangs when it reaches the read 72421.
When I remove the jumper the time/date is read correctly.
But this requires an extra manual action.
I understand that I must a chip like MAX690 to control CS1.
But since this is a rather expensive chip
(14.35 euro) I wonder if there are cheaper solutions.
I used this circuit.

+ 5v
|
1k
|
--------|--- CS1
| |
10 uF 18 k
| |
--------- 0

There is some improvement, but not all of the time.
So are there better solutions?

There is a ceramic capacitor of 100nF between
Vcc (72421) and ground. Is this enough or should a
bigger one (10uF?) be used.
Vcc is powered by a lithium battery via a diode (bat48) and by a 7805 via a 1N4148.
Is a NiCad with a resistor (1k) a better solution?

All suggestions are welcome.

Thanks,

Robert Henneke