We are sorry for this inconvenience, but we are aware of this issue and looking for a fix - the issue is unfortunately most likely connected to different fixes for shortcuts we made for other languages.
Until a patch is ready you can uninstall this version (keep database with your data) and install a previous version of eM Client 10.3, all files are available on our website: Release History | eM Client
Another workaround would be to change the shortcut for New Item in Menu>Settings>General>Shortcuts in the meantime.
If you change this shortcut to anything else but the default Ctrl+Alt+N you should be able to type ń again.
You replace a shortcut by selecting it in the list and pressing a different combination of keys you want to use, and then save the settings.