Preheat oven to 350°F and line a 5×9 inch loaf pan with parchment paper, then grease it.
Combine Greek yogurt, vanilla, melted butter, eggs, and lemon juice (and zest) in a bowl; mix thoroughly.
Add almond flour, coconut flour, erythritol, and baking powder; fold until smooth.
Pour batter into the prepared pan, smooth the surface, and bake for 50 minutes until golden.
Cool in the pan for 20 minutes, then transfer to a rack to cool completely.
Prepare glaze by mixing powdered erythritol, hot water, and lemon juice; whisk until smooth.
Drizzle glaze over cooled cake, let set, then slice and enjoy!