For everybody having the same problem:
* It happens with Debian Squeeze x86_64 as well
* PHP is compiled with dba support (not as an extension, but statically), which includes linking with libdb4. You can check this with "ldd /usr/bin/php5 | grep libdb-4.8.so"
* DBXML comes with its own libdb-4.8.so
* the two libraries are not compatible to each other, resulting in the dbxml extension using the system libdb library (at /usr/lib), which does not work
a) find a PHP release not containing dba or compile PHP yourself
b) (dangerous, only for testing) symlink /usr/lib/libdb-4.8.so to the libdb-4.8.0.so used by bdbxml