Hi. I have just downgraded to 1.9.14 and my reader appears to be working fine. I still have not tried plugging my SDHC card in, but will do so over the weekend when I have more time.
Some tips:
I downloaded 'kobo3-update-1.9.14.zip' from BigJ's link above (post #422) thanks BigJ.
I then followed mconh's instructions at -
https://www.mobileread.com/forums/arc.../t-166385.html - thanks mconh.
The extra I did was to backup my files from '.kobo' before unzipping the new file and before rebooting my reader disconnected the wifi as after reboot airplane mode is off.
A very smooth downgrade, went like a dream :-)