I have a fresh install of PS:T. I have all the latest versions of the mods recommended by the mods guide on this site, and they were installed as per the explicit instructions. The game was working wonderfully, until I attempted to use the portal to exit the Mortuary. I got a couple of millisecond image flashes, and then black screen. I had to use Task Manager to close the game.

I was eventually able to screen capture the images that flash by almost too quick to see. The first is a 'saving' screen. The second is an error message:
"Please insert Planescape: Torment disc number 255 in drive
To quit the game, press ALT+F4"
Disc #255? <blink> For one, I don't think it had that many discs in the first place, and for two, I have the GOG version so there are no discs. (And for a bonus three, ALT+F4 doesn't work to close the game, only Task Manager allows it.)

I have all the mods installed as per the instructions. I am running Windows 7 x64 with the Widescreen mod set to display the game at 1680x1050. Help, please?

You didn't by chance install the patch for the game before installing the mods, did you?

No, I didn't patch, since GOG takes care of that.

Okay, just double checking. I've seen quite a few problems posted (especially ones referencing needing a disc in the drive) on various games that were caused by people patching GOG's releases. So just wanted to try to eliminate that right out of the gate.

I should have attempted a reinstall before I posted here... guess I got ahead of myself. Will try that soon as I have time!

Edit:
The re-install seems to have done the trick. I have had a couple of pure crash-to-desktop while zoning (completely unlike the prior issue), but it seems to be, at least so far, an infrequent occurrence- so I'll just save even more regularly.

Now I get to finally play a game I've been intending to for an ungodly long time. Happiness!

Same (or similar at least) here, were you able to solve the problem?