Hello,
I have solved the problem you can’t lauch Unreal 4.26.
go in the unreal 4.26 instal folder, /engine/plugins/
You must have a 3dconnexion folder, open it and delete all the files
download here the plugin : -SpaceMouse/releases/tag/1.1.1 ,the version you need, the best is the prebuild version.
I have create a sub folder named MouseNavigator in the UE_4.26/Engine/Plugins/3dconnexion
So you have : UE_4.26/Engine/Plugins/3dconnexion/MouseNavigator
unzip de downloaded file in the sub folder you have created
and it will work very well, you just have to activate the plugin in the UE4.26
Thanks to all