In Brave, I get the following error opening mailto links:
“Unable to create io-slave. klauncher said: Unknown protocol ‘mailto’.”
Only Brave seems to throw this error; mailto links work properly in both Firefox & Chromium.
Brave Version 1.16.76 Chromium: 86.0.4240.198 (Official Build) (64-bit)
on Kubuntu 20.10 (current release version)