First, we recommend you refresh your Steam install. You can find instructions to do that here:https://support.steampowered.com/kb_article.php?ref=3134-TIAL-4638
If this does not help, try deleting your KillingFloor/system folder, and then verifying your game cache in Steam. Instructions for that can be found here: https://support.steampowered.com/kb_article.php?ref=2037-QEUH-3335
If you are using any third party mods that add characters, you may want to try removing those, before you deleting your KF/System folder as they may be causing a conflict with your recently purchased addons.