I can't say for sure why ACA is filtering out the space you're trying to create, but here's one possible workaround/test case.
Create four new walls that create an enclosed space. Try inserting a space using those walls. If it still doesn't work, repost and let us know what happened.
If it does work, select the space, and in the properties palette, change it to a "non-associative" space. Then move the white grips to relocate it to your original space. You can add or remove vertices afterwards to get it to the correct shape/size if necessary (just look up how under the help menu).
Time is too valuable to waste trying to figure out every single problem that comes along. Find a workable solution and move on. Sometimes programs and files are just screwy.
And I'm not sure why Doug suggested that you can't apply a space to a polyline in ACA 2008, because you can (unless I misunderstood what he was saying). Right-click the space tool in your palette and choose "Apply to linework or AEC object" and then select the polyline. It works best if it's a closed polyline (select it, type PE, then type C for close), but sometimes it works on open ones as well.
Good luck.
Edited by: rosskirby on Nov 14, 2009 7:59 PM
Ross Kirby
Principal
Dynamik Design
www.dynamikdesign.com