The unique link you receive via email when you approve or cancel a domain transfer or verify a domain contact is only valid once. If it's already been accessed for any reason, you will receive the (incredibly unhelpful) error message "Invalid URL." This message/page is not generated by us and we have no control over its content.
If you encounter "Invalid URL" by surprise, usually this means one of two things has happened. Either your email client has "helpfully" prefetched the URL, or, most frequently, you double-clicked the link in the email causing it to load twice. In both cases, the action you're verifying is already done.
If you're transferring a domain to us and you get this from the approval link, this means the transfer is approved and you can confirm this by going to the domain's info panel in our member interface (select "Manage" in the "Registrar" column for the domain on the Domains panel) and confirming the Action status is "Transfer waiting for Losing Registrar Approval." Likewise, if you get it from the cancel link, the domain transfer is already pending cancel and should be killed and refunded within a few hours.
If you are transferring a domain away from us, this means the transfer will be blocked.
If you're approving a contact, this means the contact is approved and that status should reflect in our UI within an hour.
If you receive this message and whatever you're doing doesn't show up as successful (i.e. the Action status is still "waiting for admin contact approval" or the domain still shows as unverified after an hour) then it's also possible your email client may be breaking up or mangling the URL. Check to make sure it hasn't split the URL onto multiple lines, possibly adding a character at the end of the first line.