When running a selenium test it fails with:

com.thoughtworks.selenium.SeleniumException: ERROR Server Exception: sessionId should not be null; has this session been started yet?
at com.thoughtworks.selenium.HttpCommandProcessor.doCommand(HttpCommandProcessor.java:73)

When inspecting the log file of the selenium server I noticed a stack trace:

21:08:32.588 INFO [11] org.openqa.selenium.server.SeleniumDriverResourceHandler - Command request: getNewBrowserSession[*iexplore, http://localhost:8280/tpm-selenium] on session null
21:08:32.588 INFO [11] org.openqa.selenium.server.BrowserSessionFactory - creating new remote session
21:08:33.804 WARN [11] org.mortbay.http.HttpConnection - POST /selenium-server/driver/ HTTP/1.1
java.lang.StringIndexOutOfBoundsException: String index out of range: -1
        at java.lang.String.substring(String.java:1938)
        at org.openqa.selenium.server.browserlaunchers.WindowsUtils$RegKeyValue.<init>(WindowsUtils.java:622)
        at org.openqa.selenium.server.browserlaunchers.WindowsUtils.deleteRegistryValue(WindowsUtils.java:577)
        at org.openqa.selenium.server.browserlaunchers.WindowsProxyManager.handleEvilPopupMgrBackup(WindowsProxyManager.java:106)
        at org.openqa.selenium.server.browserlaunchers.WindowsProxyManager.init(WindowsProxyManager.java:81)
        at org.openqa.selenium.server.browserlaunchers.WindowsProxyManager.<init>(WindowsProxyManager.java:65)
        at org.openqa.selenium.server.browserlaunchers.InternetExplorerCustomProxyLauncher.<init>(InternetExplorerCustomProxyLauncher.java:48)
        at org.openqa.selenium.server.browserlaunchers.InternetExplorerCustomProxyLauncher.<init>(InternetExplorerCustomProxyLauncher.java:41)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
        at org.openqa.selenium.server.browserlaunchers.BrowserLauncherFactory.createBrowserLauncher(BrowserLauncherFactory.java:124)
        at org.openqa.selenium.server.browserlaunchers.BrowserLauncherFactory.getBrowserLauncher(BrowserLauncherFactory.java:81)
        at org.openqa.selenium.server.BrowserSessionFactory.createNewRemoteSession(BrowserSessionFactory.java:307)
        at org.openqa.selenium.server.BrowserSessionFactory.getNewBrowserSession(BrowserSessionFactory.java:113)
        at org.openqa.selenium.server.BrowserSessionFactory.getNewBrowserSession(BrowserSessionFactory.java:78)
        at org.openqa.selenium.server.SeleniumDriverResourceHandler.getNewBrowserSession(SeleniumDriverResourceHandler.java:653)
        at org.openqa.selenium.server.SeleniumDriverResourceHandler.doCommand(SeleniumDriverResourceHandler.java:410)
        at org.openqa.selenium.server.SeleniumDriverResourceHandler.handleCommandRequest(SeleniumDriverResourceHandler.java:388)
        at org.openqa.selenium.server.SeleniumDriverResourceHandler.handle(SeleniumDriverResourceHandler.java:135)
        at org.mortbay.http.HttpContext.handle(HttpContext.java:1530)
        at org.mortbay.http.HttpContext.handle(HttpContext.java:1482)
        at org.mortbay.http.HttpServer.service(HttpServer.java:909)
        at org.mortbay.http.HttpConnection.service(HttpConnection.java:816)
        at org.mortbay.http.HttpConnection.handleNext(HttpConnection.java:982)
        at org.mortbay.http.HttpConnection.handle(HttpConnection.java:833)
        at org.mortbay.http.SocketListener.handleConnection(SocketListener.java:244)
        at org.mortbay.util.ThreadedServer.handle(ThreadedServer.java:357)
        at org.mortbay.util.ThreadPool$PoolThread.run(ThreadPool.java:534)
21:08:33.804 DEBUG [11] org.mortbay.http.HttpConnection - RESPONSE:
HTTP/1.1 500 Internal Server Error
Date: Wed, 07 May 2008 19:08:32 GMT
Server: Jetty/5.1.x (Windows XP/5.1 x86 java/1.6.0_04
Connection: close

I think line number 106 of WindowsProxyManager shoul be changed to:
WindowsUtils.deleteRegistryValue(RegKey.POPUP_MGR.key);