What does this mean: "mds: (Warning) FMW: event:1 had an arg mismatch. ac:2 am:51"
These are undocumented internal error conditions and most people outside of Apple can't say for sure what they mean. I wish I had a better, more concrete answer, but here are some ways to poke at your system to get a hint at which file is causing these errors to be emitted...

mds == indexing service. rule of thumb == don't f**k with it unless you know what you do. 


Have you reset the spotlight database using any command line tools? (mdutil or mddiagnose) or conventional methods adding/removing directories from the privacy pane in System Preferences?
Does mdutil -s -a
-v say anything useful? 
Disabling dropbox may help narrowing the cause


Lastly, watching for other mds related console output might be helpful.