Just a few minutes ago Urban Terror 4.2.20 was released with a rather long list of bug fixes to the new mod introduced in 4.2.19, Freeze Tag.
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4.2.20 features over 20 bug fixes, and most of the fixes or improvements in this release are concerning only the Freeze Tag mode, with some of the most notable changes being a scoreboard fix, frozen players??? hitboxes fix or a bugfix where players were shown up as frozen on new round.
If you already have Urban Terror installed, you just need to run the updater (called either UrTUpdater.i386 or UrTUpdater.x86_64, depending on your architecture). If not, you can go on and download the full game (which includes binaries for Linux, Windows and OS X) from the downloads page – the file is a ZIP archive called UrbanTerror42_full020.zip.