This is a known issue with AppleGuide 1.2.5, installed by System 7.5 Update 1.0, on Power Macintosh computers. Mixins installed for features that are not present in the computer (like AV Mixins installed on a computer which has no AV technology) will cause Apple Guide to "lose contact" with the Macintosh Guide and Tutorial files, thus causing all of the Guides to no longer work.
A "Mixin" is an Apple Guide content file that merges with the main Apple Guide file to add extra options. On any Macintosh computer running System 7.5 there is a standard Apple Guide file and there may also be another Apple Guide file added to the system depending on the features of that particular Macintosh computer.
The solution is to remove the extra unnecessary Mixins that do not belong on your Macintosh computer.
The only way to mistakenly install Mixins that are not needed by your computer is to do a Custom Install of the System 7.5 Update 1.0 for ANY Macintosh. An Easy Install or Custom Install for your particular Macintosh computer will not install unnecessary Mixins.
This issue is documented in the System 7.5 Update 1.0 Read Me file. The following text is an excerpt from that ReadMe file.
Problem with Guides on Some Power Macintosh Models
If you have a Power Macintosh and have trouble opening the Tutorial, Macintosh Guide, Shortcuts, or other Apple Guide databases, you may have extra files that don't belong on your computer. To solve the problem, follow the steps below:
1. Open the Extensions folder (inside the System Folder) and drag the PowerBook Guide Additions file to the Trash.
2. Unless your computer is an AV model, drag the following items to the Trash:
* Video Guide Additions
* Video Player Guide Additions
* Speech Guide Additions
3. Choose Empty Trash from the Special menu.
4. Restart your computer.
Other Guide Mixins from Apple or third party applications may prevent the Macintosh Guide from opening. In some cases, you may need to choose which Guide is more important to you and remove any extra guides which are causing the problem.