hello!

oh man... all of you were right, the LPC92x-series is fully compatible. unfortunately i found a slight differenc between them.

the mistake was in the serial-communication i used. as i already mentioned, i use a slightly modified circuit of the MCB900-evaluation-board. this means, that i don't use the max232, i use a V24-module instead. it's a level-converter, which converts the RS232-levels into ttl-levels and vice versa; one signal in each direction, for RxD and TxD. the whole V24-module is a pretty dirty solution and it doesn't need any capacitors - but it works.
the V24-module needs a pull-up on the microcontroller's TxD-line to pull it up to 5V - this worked with the LPC922, but didn't with the LPC925. it seems as if the TxD-pin is configured differently in the bootloader. the LPC922 used either open-drain or quasi-bidirectional, but the LPC925 configures TxD as push-pull. therefore the 1-level isn't 5V anymore, it's just 3.3V. and this is too less for the dirty V24-module. it just didn't generate any valid signals anymore.
now i replaced it, and built the circuit with a max232 and violá, it worked!

thanks for your great effort and help

greets, christian hartl