Just checked GMAIL, activation worked perfectly fine

FRom the DEveloper:
There is a problem with the links in our e-mails in the current version which doesn't work under specific circumstances. When requests comes from e-mail-addresses, managed by an Exchange server with activated Safe Links feature, the request will fail in a few of the e-mails. This is, because the Safe Links feature tests the connection to find out if the link is possibly malicious with a HTTP method, which is not supported by the MLF scripts but will be accepted by the script.

The solution in one of the replies to the report was, not to click the link in the e-mail but to copy the URL and to natively open it in the browser. The next release will include a fix for the problem.