If you read the bottom, of the download page, you will read that you need to set a password, in your VU+ receiver, for some applications, like Filezilla, DreamboxEdit, DCC, (Dreambox Control Centre), VUCC, (VU+ Control Centre, etc.
This shouldn't apply to Openwebif, so most likely your receivers IP Address has changed, just check under Network, on your receiver, to confirm your receivers IP Address.
While there you can also set a password. Don't forget to save it, when exiting.
Once done put the password into the required utilities, instead of leaving the field blank.
If you still can't access Openwebif, after checking your receivers IP Address, please try a different browser, like firefox, and if it still fails check your security settings.