. What options did you specify on Export of connections?
. Were the connections exported from the same SQL Developer release as you are trying to import to?
. How was the file copied from users machine to your machine?
. Was the xml file encrypted?
. Is the file corrupt?
Try recopying and make sure it is identical to the source file.
it's a bug
upgrade to version 18.2
and then upgrade to version 18.3
this has been fixed for 18.4