Hello,
I did some further testing (with my means). Unfortunately I am not an expert in this.
I created a script which logged the following output:
Wasserzahlerlesen_Ruckgabewert.historicState(now.minusHours(1), "rrd4j")
I ran it serveal times and it always gave me the correct answer, never “NULL”.
Meanwhile I have found this thread which seems to have (solved?) a similar issue: deltaSince or historicState sometimes returns null on openHAB 3.2 - #12 by rossko57
Thanks again for your support!