Problem solved. Thanks for reacting so quickly!
Are you still getting the issue today? We had a technical issue on one of our servers yesterday, which is now resolved, so this may have been causing the problem.
Similar to issue - GPX and KML files trying to upload and getting "Unable to read file" as well.
When uploading gpx-files previous made on Plotaroute it uploads 99% and states "Unable to read file". Only saved gpx-files made today can be (re-)uploaded succesfully. What can be wrong??