In TRAMP 184.108.40.206, the sshx and scpx methods were changed to use RemoteCommand for specifying, well, the remote command to run (see commit aaffd34492ea6168cc47c469799ac87aa5c052a9 for the actual change). However, this was specified as
RemoteCommand='%l', which fails on MS Windows systems since they only allow shell-quoting via double quotes.
I've confirmed locally that changing to double quotes, i.e. RemoteCommand="%l", makes everything work correctly. Obviously that isn't what you'd want on a *nix system, but even on MS Windows, I imagine this would fail for multihop cases (e.g. MS Windows -> Linux -> Linux). There might also be further issues with quoting if the %l placeholder is generated with sh-style quotes, but I haven't encountered any problems like that.