This addon is not being updated any more, but there is an excellent replacement. Delete all Spellflash addons, install Spellflashcore, and switch to Ovale. From SIL3ENTcurse's post below:
1: REMOVE SPELLFLASH AND ALL ITS ADDONS.
2: Install Spellflashcore https://mods.curse.com/addons/wow/spellflashcore
3: Install Ovale https://mods.curse.com/addons/wow/ovale
I did this and it's working great. Took a little tinkering to get the flashes to where they worked for me, and you have to make sure to delete ALL Spellflash addons before installing Spellflashcore and Ovale.
That's it. Flashing spells are back!