Jenny at Adobe helped me out - there's a link to this issue:
http://www.adobe.com/go/rb_34906
TechNote
Opening a Corrupt HTML Help Project
Issue
Your project file is corrupt and cannot be opened.
Solution
RoboHelp X5 instructions:
Follow the steps below to recreate the *.XPJ and *.CPD file,
which should resolve the problem:
1. Close RoboHelp HTML.
2. Create a backup of your project folder, and all its
contents before following the remainder of the instructions below.
3. Open your project folder, Windows Explorer.
4. Find the *.XPJ and the *.CPD files, which are located in
the folder.
5. Delete the *.XPJ and the *.CPD files.
6. Find the *.HHP file (in the same folder as the files
listed above).
7. Right-click on the *.HHP file and choose the EDIT option.
This should reopen the project in RoboHelp HTML.
8. Save and compile the project.
Note: Avoid moving files outside of RoboHelp and using
characters other than A-Z, 0-9, or underscores in your filenames or
folder names. Other causes include renaming files outside of
RoboHelp, moving RoboHelp project folders or files outside of
RoboHelp, having more than one help author work on the same project
at the same time, working on a project from a network drive, and
opening a project over a UNC path, otherwise you may experience
corruption again in the future.
RoboHelp X4 and below instructions:
Follow the steps below to recreate the .MPJ file which may
resolve the problem:
Create a backup of your project folder, and all its contents
before following the below instructions.
1. Close RoboHELP.
2. Open your project folder.
3. Find the .MPJ file.
4. Change the file extension of that file to .MPJ_OLD.
5. Find the .HHP file (in the same folder as the .MPJ).
6. Right-click on it and choose the EDIT option. This should
reopen the project in RoboHELP. If it doesn't, then you'll need to
open RoboHelp HTML and browse to the HHP file.
7. Save and compile the project.
WARNING: Following these steps for rebuilding the .MPJ file
will cause the .ALI file to be recreated as well. The .ALI file is
used for setting up context sensitive help and if this file is
recreated, all of the data it contains will be deleted. Before
following these steps, backup your .ALI file to a different
location on your hard drive. Once you have rebuilt the .MPJ file,
copy the .ALI file back to the project folder and overwrite the new
version of this file.
WARNING: By recreating your MPJ file you will lose any UNUSED
bookmarks, Topic Status Information, and Build Expressions in your
project.
Note: Avoid moving files outside of RoboHelp and using
characters other than A-Z, 0-9, or underscores in your filenames or
folder names. Other causes include renaming files outside of
RoboHelp, moving RoboHelp project folders or files outside of
RoboHelp, having more than one help author work on the same project
at the same time, work on a project from a network drive, and
opening a project over a UNC path, otherwise you may experience
corruption again in the future.
TechNote Details
Last Update: 03-20-2007
ID: rb_34906
Permanent Link:
http://www.adobe.com/go/rb_34906
Products Affected:
* RoboHelp HTML