Code:
/Users/pdurrant/Documents/Projects/Sigil/Sigil-0.6.2-Code/src/Sigil/Misc/FontObfuscation.cpp(113):
What are you doing with pdurrant's Mac?
Right click on the font files (in the Fonts folder) in the Book Browser and see if either option for obfuscation (idpf or adobe) is checked. If so, change it to 'None' (for all fonts present) and see if you can save it then.