TMPGEnc 4 XPress doesn't like AVISynth anymore

[pintcat]

Recently I tried to import a video into TMPGEnc XPress via an AVISynth script as I did a million times before, but suddenly TMPGEnc is unable to open the video.
I double-checked everything - the script, the video itself, TMPG's configuration... I even re-installed TMPGEnc and AVISynth, but nothing helped.
I then did some tests with other videos - nothing special, some divx & xvids which could be opened by TMPGEnc without the help of AVISynth. But if I use a script to import them, TMPGEnc fails again.
The scripts I use for testing were very simple:
AviSource ("D:\testvideo.avi")
or
DirectShowSource ("D:\testvideo.avi")
They work flawless in VirtualDub, MPC, MeGUI and other programs which support AVISynth, but not in TMPGEnc. The funny thing is if the script contains a typo or anything that prevents AVISynth from working properly and makes it come up with one of these red messages, TMPGEnc is able to open and display this message.
At this point I'm totally out of options. Does anyone have a clue what might cause this behaviour?
My system is WinXP SP3; TMPGEnc version is 4.7.7.307 (but tried earlier versions too); AVISynth version is 2.5.8

[Guest]

What error message do you get? Specifically, how does it fail?

[pintcat]

It just says either "Could not open the video file D:\test.avs" or in some cases "Could not open the video part of the file D:\test.avs. Only the audio part could be opened." It doesn't crash or something like that.

[pintcat]

Finally found the culprit. I had to activate the 'DGMPGDec 1.5.8 D2V/AVS Reader' function in the preferences to enable AVISynth support. This option wasn't available until I started DGMPGDec the first time which is something I don't understand: Do I need to have DGMPGDec available to make TMPGEnc XPress AVISynth-capable?