How do I resolve this error?
This error indicates the friendsUI.dll file is corrupted or outdated.
The friendsUI.dll is extracted from the winui.gcf found in your ..\Steam\steamapps\ directory. To ensure you have the most recent working friendsUI.dll, please delete or move the winui.gcf out of the steamapps directory and restart Steam. The process will force Steam to download the latest version of winui.gcf and re-extract your friendsUI.dll.
If deleting winui.gcf does not correct the issue, you must delete all instances of friendsUI.dll to allow winui.gcf to re-extract the file: Run a search for friendsui.dll in your Steam directory and delete the files, then restart Steam.