I just went up a level when i completed achievement 6/8 in Hero of Terror. Initial level up worked, then i got ERROR: Reload game. When I reloaded I had lost all the energy and the double up on the gems although it is now showing 7/8 for achievment.
Contact Pixel Support, they can check what happened.
Open the settings menu (⚙️) in your game and push the SUPPORT button.
In the support page scroll down and choose a category.