I used export selection & chose TCX for the first time & found that I only got headers & no data. The total content of the file is quoted below
"<TrainingCenterDatabase xsi:schemaLocation="www.garmin.com/.../v2 www.garmin.com/.../TrainingCenterDatabasev2.xsd" xmlns="www.garmin.com/.../v2" xmlns:xsi="www.w3.org/.../XMLSchema-instance" xmlns:tp1="www.garmin.com/.../v1" xmlns:gpx="www.topografix.com/.../1"><Activities /></TrainingCenterDatabase>"
exporting as a GPX worked fine.
I realised that I think this has happened before when someone sent me a route, created in BaseCamp, via email last year & I found that the TCX they sent was similarly truncated but the GPX they also sent was fine.