The prototype you are looking for is called Neos.Neos:ContentElementEditable.
Very likely your code only uses ContentElementEditable and was autocompleted to Neos.Neos in the past via Fusion namespace.
Background: Ages ago default namespaces were used in included files aswell. This was changed and now namespace declarations are only valid for the fusion file they are defined in. Otherwise the default namespace is Neos.Fusiom which is probably the reason of your problem.
I recommend to always use full prototype names and an ide that supports autocompleting.
Regards, Martin