Error accessing entry when title has special characters
I've never used that plugin, but it looks like they do respect Craft's limitAutoSlugsToAscii config setting, which should prevent a % from getting into the slug.
https://github.com/internetztube/craft-slug-equals-title/blob/master/src/SlugEqualsTitle.php#L45-L47
But that setting only applies to new entries and newly created slugs, and I'm guessing you enabled the Craft limitAutoSlugsToAscii config setting after the entry in question was already created.
You should be good going forward, though.