Well, I found out that simply adding a CheckBox to the form, breaks it. Remove any checkboxes, and everything works fine.

Hello Terje! There is currently a bug that clears the value input box when opening the properties for a multiple options input if there is only a single alternative. If the field is saved without reentering the value the form will be parsed incorrect and you will get the error. It's still possible to add check-boxes but you will just have to reenter the value(if you only have one value) if you have to edit the properties for the field. This bug is fixed in EPiServer 4.51. There is also a bit more trickier bug that can appear if you enter a value including the ' character as this breaks the editors javascript. Unfortunately we have not fixed this bug in 4.51.

Thank you for your answer Linus. So, if I understand things correctly, it is impossible (as of 4.50) to add check boxes or radio buttons that do not contain alternatives. (In other words, it is impossible to create these components without adding text to them)? Please feel very free to correct me if I am wrong here, I am hoping I am :)

I'm sorry to say that it is not possible to add check boxes or radio buttonts without any alternatives in 4.50 and this is a feature "by design" and not likely to be changed. We do, however, have a feature request that is to split the value and the text shown so this might fix your problem but I can't say when we fix this although it has a pretty high priority.