experimenting testing TFS 2013 Update 2 over the last few days, and encountered a couple of issues.
The first appears to be a problem that is specific to a Team Project Collection where the Activity Log (accessed via _oi) does not render correctly (progressing this on with MS Support), however, the second one is slightly more interesting.
If you go into the /_oi interface, select Job Monitoring, pick a job, you go to the detail page. Only now you also get something unfriendly.
Confirmed this on a clean install of Server 2012 R2, with TFS 2013 Update 2, so it seems that this is a breaking "change".
Hopefully a hot fix comes out soon for this one (and maybe the Activity Log issue).
Update: Just as I'm posting this, I get a call back for MS Support. Both issues confirmed as bugs, and will be fixed in a future release. Issues that you might encounter with the Activity Log should automatically clear up after a month or two as old entries are purged - so if you encounter a TF400898 here you will probably have to put up with it for a while!