If you receive an error message when trying to access a WeTransfer page like 'Your connection is not secure/private' or 'Invalid certificate' mentioning HTTP/HTTPS, it's likely to do with the formatting of the URL.
The fix
To fix this, simply remove 'www' at the start. For example, instead of www.support-test.wetransfer.com you should be visiting support-test.wetransfer.com.
Why this happens
'www.' at the beginning of a URL indicates a website is being hosted at that domain. With a WeTransfer page, most Profile URLs act as a subdomain — doing the same job as 'www.' does with regular websites.