Hallo Alexej,
Still no real answer on your question, but I will try to help if you can describe exactly the circumstances.
the contents of the sfr :
IE
IP
PCON
WDT
Etcetera.
A big help can be the application notes from Philips.
Philips.com go to semiconductors=>microcontrollers+. 8051 based=>application notes and search fot watchdog. This is really a good site with lots of detailed info.succes

Robert