Thanks for the answers.
Maybe I should illustrate it on an example.
Client request is: https://www.example.de/external/
ProxyPass is rewrite: /external/ := / /pool_backend
In some cases the backend system is using absolute path or someting else and the next client request is without the /external/ path (it will replaced by the absolute pathes). So the ProxyPass can't handle these requests because there are no matches in the DataGroup of /external/ anymore.
My idea was to modify some or all http responses from the pool_backend system(s) to identify all new requests on it. In some cases I am able to identify it on the Referer(field) in the request header but not on all.
Thanks