Just a quick note that running "mono Fiddler.exe" (Fiddler 4.6.20171.14978; Linux Mint 18.1 32-bit; mono-complete installed; mono certificates synchronised) produces the following output on the command line:
can not parse as value of type 'i':
""
^^
can not parse as value of type 'i':
""
^^
The program itself seems to run fine, in my limited testing.
As a side note, is it possible that you could make the version number in the Help window selectable and copyable to make it easier to paste the version number here?