One thing you might try is to pop off the space bar key and make sure it that there is not something blocking it from making contact such as dirt, lint or spill etc.
You did not post what type of keyboard you are using so I am assuming it is an Apple Pro type keyboard which you should be able to use something like a small screw driver to wedge under the ends of the space bar and gently pry it off. If I remember correctly the space bar has two connections at ends that need to be pried off. It usually is very easy to pop off but if you have any difficulty do not force it.
One other thing is that you did not post if you have tried another keyboard (if possible) to test?
post back with comments or questions,
littleshoulders 🙂
PS. Discussions, as you may be aware, are having many performance and stability problems these past few days which may make it very difficult to post/reply...be patient and we will try to post back to help if we can.
I have the same problem: White ApplePro-Keyboard with wire and Spacebar defect.
The keyboard works at other computers, but at this PowerMac the Spacebar doesn't work sind some days. With another User-Account it doesn't works, too. It seems to be a software-bug.
While problems can sound the same on the surface there can be different causes so you might want to post you question in the Tiger 10.4 Usage forum if you feel it is a software related problem.
This forum is for 10.3 and earlier usage. For the best help for your problem you should post in that forum.