Is there a new incompatibility between ragged2e an scrlttr2?

My suggestion is to “robustify” \@centercr (which is the meaning \\ is given under \raggedright).

\documentclass{scrlttr2}

\makeatletter
\MakeRobust\@centercr
\makeatother

\raggedright

\begin{document}

\begin{letter}{x\\ x \\ x}
\opening{Hi,}
Text
\closing{Bye}
\end{letter}

\end{document}

It's the same if you use \RaggedRight, because it uses \@centercr as well.

The KOMA-Script bug is fixed in the current prerelease of the next KOMA-Script Version (will be 3.27a):

\documentclass{scrlttr2}[2019/10/28]% needs at least version 3.27a.3319
\usepackage{lipsum}% only for dummy text
\usepackage{ragged2e}
\RaggedRight
\begin{document}
\begin{letter}{ x \\ x \\ x }
\opening{Hi,}
Recipient: \usekomavar{toname}, \usekomavar[\def\\{, }]{toaddress}

\lipsum[1]
\closing{Bye}
\end{letter}
\end{document}

Possible workarounds for KOMA-Script Version 3.27:

\documentclass{scrlttr2}
\usepackage{lipsum}% only for dummy text
\usepackage{ragged2e}
\AtBeginLetter{\RaggedRight}% <- changed
\begin{document}
\begin{letter}{ x \\ x \\ x }
\opening{Hi,}
Recipient: \usekomavar{toname}, \usekomavar[\def\\{, }]{toaddress}

\lipsum[1]
\closing{Bye}
\end{letter}
\end{document}

or

\documentclass{scrlttr2}
\usepackage{lipsum}% only for dummy text
\usepackage{ragged2e}
\RaggedRight
\begin{document}
\begin{letter}{ x \\ x \protect\\ x }% <- changed
\opening{Hi,}
Recipient: \usekomavar{toname}, \usekomavar[\def\\{, }]{toaddress}

\lipsum[1]
\closing{Bye}
\end{letter}
\end{document}