Whenever i want to download a Mod, for an example the NRaas Traffic one, i get a blank page with this error
"{"error":{"number":500,"message":"The file browser is disabled for security reasons. Please contact your system administrator and check the CKFinder configuration file."}}"

Did you try another browser?
Or do you brows in private mode? Maybe this causes it.

Does this also happen when you right click the download link and choose "save target as"?

It sounds like you're trying to access the file manager when you aren't logged in. You do not need to access the file manager to download. Just use the green download buttons on the mod pages themselves.