Normally at work I use my laptop with docking station and external monitor connected to it. However, I need to undock the laptop a couple of times a day when I go to meetings and also at the end of the day. Each time when docking back the external display gave just an error message about wrong display mode and it did not start working without rebooting no matter what display settings I tried. External monitor is also Dell's panel so it should not be a question of incompatibility (and it is detected nicely after reboot). I tried updating all drivers which could have any effect on this, and BIOS was already the newest version, but the problem continued.
Then by chance I bumped into a solution (not final, though): When the laptop was in docking station I changed the display mode to "Only projector" (Windows 7: Control panel->Display-> Connect to a projector). Obviously both the laptop's screen and external monitor were now blank, but when I closed the lid and let it enter sleep mode and then pressed standby/power button in E port for coming back from sleep mode - voilá, the external monitor started working!
So, it seems that Dell's drivers are not able to detect or communicate with Windows about the correct display mode if it has been changed between undock/dock. And do not anyone dare to trivialize this as if it would be "acceptable" for all laptops to have display problems with docking stations: I have never had these kind of problems with IBM/Lenovo Thinkpads although I have had numerous of them before Dell, and also with the same Windows version.
I finally got the problem to disappear, although it is not really a solution but a testimony to the unreliability that Dell obviously has with E-port: I changed the original single link DVI-D cable to dual link cable.
Now, the funny thing is that 2001fp supports only 1600x1200 resolution, so it really SHOULD NOT NEED DUAL LINK CABLE! Notice also that the single link cable works perfectly if I reboot the laptop while connected in E-port, so there is nothing wrong with my cable (and also Thinkpads work perfectly even with docking station with the same single link cable). It is just that there is some problem with E-port and display mode detection in Dell which for some reason does not show up with dual link cable. I really would like to get some answer from Dell representative WHY ON EARTH DELL NEEDS DUAL LINK CABLE ALTHOUGH THERE IS NO TECHNICAL NEED FOR IT?!?