I have tried to export all my data from the iOS app.
I retried today, with a new export from the web app.
Each time I get a link by email, where I just have to press on the button to download the csv.
In both case, the downloaded file is a zip of only 2 bytes. (Which is of course wrong seeing the amout of data I have :-)
Download with Firefox 86.0 on Mac OS 11.2.3