I have seen two Windows 10 1909 with latest Libreoffice machines now where “Send as email” from LibreOffice doesn’t work. LO puts up this “Please make sure that the email program is your standard app” message.
All the machines are running latest Thunderbird.
I tried this on a test machine running Win10 1809. First with LO 126.96.36.199. It worked!
Uninstalled 188.8.131.52 and replaced it with 184.108.40.206. Still working!
I cannot really see any differences (except Windows version) between these machines.
Thunderbird is standard email app in them all.