I'm looking for a solution for this same exact issue as well.
I'll post anything additional that I find.
So, in addition wikid/collabd not working, I was also seeing gobs of error messages in /var/log/system.log from sandboxd regarding ruby:
Apr 17 11:34:07 reddwarf sandboxd[3890] ([3887]): ruby(3887) deny
file-read-metadata /private/var/folders/zz/zyxvpxvq6csfxvn_n00000vm00006x
I found a thread in Apple Communities (https://discussions.apple.com/thread/3216408?start=0&tstart=0) that showed a possible solution:
Can you uninstall the newer version of Rack, 1.2.2?
sudo gem uninstall rack
sudo gem install rack --version 1.2.1
This is a bug we're aware of.
I decided to do this since I had originally got mired in this mess after updating ruby.
First off, stop the wiki server...
sh-3.2# serveradmin stop wiki
wiki:state = "STOPPED"
I found I had multiple versions of rack installed (1.4.1 and 1.3.5), but 1.2.1 did not show as installed, which evidently is the version of this gem that wikid depends on.
sh-3.2# gem uninstall rack
Select gem to uninstall:
1. rack-1.3.5
2. rack-1.4.1
3. All versions
> 2
I uninstalled both of these, trying to start the wiki server after each was removed. The wiki was still not working up to this point. Then I installed 1.2.1...
sh-3.2# gem install rack --version 1.2.1
Successfully installed rack-1.2.1
1 gem installed
Installing ri documentation for rack-1.2.1...
Installing RDoc documentation for rack-1.2.1...
After that, another attempt at starting the wiki...
sh-3.2# serveradmin start wiki
wiki:state = "STARTING"
sh-3.2# serveradmin status wiki
wiki:state = "RUNNING"
And guess what? It actually worked! My wiki is now functional again.