I think I have found the trigger for the
German popularity of the name Rabea in the late 1970ies:
It is Rabea
Hartmann, assistant to TV host
Wim Thoelke in the TV show Drei mal Neun (ZDF) aired from 1970 bis 1974. She was also assistant to
Uri Geller in 1974.
I don't have information how she got her given name.
--elbowin
P.S I discard the theory that it is a variant of Rebecca—I have never heard of similar sounding variants of
Rebecca before, and
Rebecca became only popular in Germany in the late 1960ies.