Unfortunately as Tony already mentioned it's a bug that hardlocks you :/ Nothing much you can do but reload the page. Really sorry that you're experiencing this issue (But pss if you don't want to redo all the fights, the button on the right of L on your keyboard (ö on a german keyboard) can be used to skip bosses, don't tell anyone).
But I'm glad that you're loving it <3 we really poured in all our effords in this project!