hints file based rewrite does not work (migration from freeradius 1.1.x to 3.0.x)

[Bernd]

Hi list,

we're about to move from a freeradius (freeradius-1.1.3-1.6.el5 on 
CentOS 5) to a HA freeradius (freeradius-3.0.4-6.el7.x86_64 on CentOS 7) 
environment.

We're almost there, however, I ran into an issue: I have to rewrite an 
"%" character to "@". My 'hints' file on the old installation did this 
perfectly well:

DEFAULT User-Name =~ "bla\-bla\/([^%]+)%kroenchenstadt.de"
         User-Name := "%{1}@kroenchenstadt.de"

However, freeradius BLA throws an error (running as 'radius -X' for 
debugging purposes):

(0) ERROR: preprocess : bla\-bla\/(^%)%kroenchenstadt.de
(0) ERROR: preprocess :              ^ Invalid variable expansion

Any idea what goes possibly wrong here? How could I circumvent the 
problem?

Thanks,

Bernd