documentclass{memoir}

usepackage{xcolor}

newfootnoteseries{E}
renewcommandthefootnoteE{textcolor{red}{Roman{footnoteE}}}
leteditornote=footnoteE

begin{document}

Text%
footnote{Footnote paragrah 1.
% -- removing this is fine
Footnote paragraph 2.}%
editornote{Editor paragraph 1.
% -- removing this breaks
Editor paragraph 2.}

end{document}

Runaway argument?
{rule z@ footnotesep ignorespaces {@nameuse {foottextfontE}EditoETC.
./editornotes.tex:17: Paragraph ended before @makefntextE was complete.
<to be read again>
par
l.17 Editor paragraph 2.}

?