Hi,
If one User on the Mac has an issue using the app then the fault lies with that Users Set up.
This could be .plists for the App.
It could be whether the User has Read and Write permissions of those files and the Folder that contain them.
In the case of Yosemite this includes a Read & Write "route" or Sandboxing that protects the rest of the system from issue with the app.
The issues can be with the way the Mac is set up.
DNS server info in System Preferences > Network > Advanced Button > DNS tab should be external to your LAN and not the router's IP address.
The Date and Time settings should use the Automatic Time Server.
We mentioned the AppleGothic Font but others like Helvetica Neue and Lucida Grande also need to be enabled.
As well as checking those setting do this:-
In a Finder Window use the Go Menu whilst holding down the ALT key and selecting the Library that appears.
In the View Menu select the Show Path Bar.
Now navigate to /Containers/com.apple.iChat/Data/Library/Preferences
(you will see most of the .plist here are Aliases - they point to the actual files in ~/Library/Preferences)
Right Click on the Preferences icon in the Path Bar and select Get Info.
In the Info Panel scroll to the bottom.
It should list the Name next to the Little House Icon in the Path Bar (shortname)
This should have READ and WRITE permissions.
If you select the Line and then use the cogwheel type icon under the list it should have Greyed out Text saying "Make shortname (me) the Owner" - it will be greyed out if you already own it.
Repeat the above panel part for the Library that is before Preferences and the Data and com.apple.iChat and Containers Folders.
Then click the Containers icon so you are in the folder.
Navigate to com.apple.soagent/Data/Library/Preferences
Repeat the panel bit for all .plists that are not Aliases (soagent.plist and all the ones with Messages Helper in the name)
Then again for the Preferences, Library, Data, com.apple.soagent folders.
This is long winded of course but you may find the offending item early.

8:21 pm Wednesday; January 21, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad