Which mod is causing this?

If you don't know, try disabling them all and enabling them one by one until you get the error.

I did, but I still get that error. Even creating a new folder gives me the same error. The game .exe used to create the custom folder works, though.

Please e-mail me the diagnostic report (Help->Diagnostics). davidc (at) this domain.

And... that's done.

The file "properties/text.xml.bin" in your original World of Goo directory is corrupted. Its size is 234,969 bytes which is not a multiple of 16.

I believe the original World of Goo text.xml.bin is 234,976 bytes. Please reinstall World of Goo or download a new translation file from http://hell.student.utwente.nl/wog/download-text.php

-davidc

Thanks, it worked. Now, how can I fix WOGEditor? When I double click it to run it, it starts and closes in about a second...

Try starting it from a command prompt so you can see the error that it's giving.

If you know how to edit your registry, delete HKEY_CURRENT_USER\Software\WOGCorp to reset it.

I tried, and here's what happens:
"Traceback
File "wogeditor.py", line 979, in
File "wogeditor.py", line 467, in __init__
File "wogeditor.py", line 484, in _reloadGameModel
File "wogeditor.py", line 142, in __init__
File "wogeditor.py", line 172, in _loadTree
File "wogfile.pyc", line 51, in decrypt_file_data
ValueError: Input strings must be a multiple of 16 in length.

Looks like one of your files aren't validly encrypted or is corrupted. Do a reinstall (non-modded) or let GooTool rebuild your modded installation by deleting the modded WoG program folder.