Comment # 11 on bug 1178596 from Jeff Stern

I too encountered the

Unable to create io-slave. klauncher said: Unknown protocol 'mailto'.

cryptic error.

However I was lucky enough to find a solution posted in a (more fruitful) forum
discussion and the solution posted by user "abruenin" (post #5)..

https://forums.opensuse.org/showthread.php/544819-mail-link-on-any-browser-Unable-to-create-io-slave-klauncher-said-Unknown-protocol-mailto?p=3007927#post3007927

.. also worked for me: systemsettings5 --> Applications --> Default
Applications --> Email Client --> There are 2 Thunderbirds, so choose the one
not currently selected.

I saw 2 Thunderbirds (spelled exactly the same, and no underscores, "_").

Already selected was the 2nd (lower). When I selected the 1st (upper), now the
mailto: linking worked.

Curious, I wanted to see what would happen if I returned to select the 2nd.
After doing so, the mailto: linking broke again. So I re-selected the 1st, and
again, the mailto linking worked.

(I have rebooted the machine since then, and now I see that only 1 Thunderbird
choice appears in the Email Client drop-down list.)