Hi IfAndOr,
You are right. I'm the last one to push technological boundaries but it does seem to be a limit! I have unshared all tracks and tried to download the new Watkins Glen and got "user not found", then:
"We’re sorry, the page you are looking for was not found.
Error Code: 418"
I deleted a track entirely and was then able to download as normal.
Thanks very much for sorting this out, I will be more discerning in the future!