Hi Finn,
Sorry for the delay in getting back to you and for the inconvenience you are currently facing.
Have you managed to use the CJTe Developer at all? or does it simply show a blank screen when you install activate it alongside the CSS & JavaScript Toolbox?
Also, do you have any caching plugins? have you refreshed you browser cache, and are pulling any new scripts, etc?
Do you have the latest version of CJTe Developer, which I believe is 0.6?
Thanks,
Damian