If you clear the error log and visit the contact firm, does that error come back?
Make sure you have this in the <head> section of the page template:
<base href="[[!++site_url]]" />
Also, make sure the "thank you" page is published.
[Update] I just tested and it works for me with MODX 2.1.5 either logged in or not. Actually, it only works for me with the *uncached* version.
BTW, SPForm 3.2.0-rc2 is available. I don't think it will fix your problem, but it can't hurt to try it. I'd suggest uninstalling and removing the current version first (right-click on it in Package Manager and select "Remove Package"), since you're having trouble with it.
[ed. note: BobRay last edited this post 14 years, 10 months ago.]