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/etc/Muttrc expects urlview to exist on the machine. It's not installed by default, or at least it wasn't when upgrading 5.2 to 6.0. It would be nice if it was installed, or if mutt tells you it would like it. (Leave it in the default muttrc; it's dead handy for using on a text-based monitor!) Whilst I'm at it, there's a typo in that /etc/Muttrc. I'm sure you mean 'simulate' (not 'simluate') in the urlview bit...
fixed in mutt-0.95.7us-1, which will be in next Raw Hide release...