I am running v5.75.8238
Registering under Options > About > [Register] is free form and only works temporarily until I restart CCleaner at which point upon launching it either quietly reverts to UNREGISTERED or pops up a notice that 'The registration information you entered is invalid. Please re-enter your details' with a dialog that accepts only license keys in the old XXXX-XXXX-XXXX-XXXX-XXXX format. It looks like this (the dialog is pre-populated with an old key in it that happens to be the old expired key from last year, which I've replaced with XXXXs - registering the new key via About does not influence the fact that it always comes up and says the key is invalid and replaces it with the old key in the old format).
.. I don't have any confidence in support handling this, they never fixed the problem that I can't retrieve the key when I contacted them the last time, and don't seem to be responding at all today. Payment went through Cleverbridge AG and their T&Cs contain remedies for defects that may oblige them to fix this or provide a prorated refund, since that's a contract between me and Cleverbridge AG as opposed to the EULA which is between me and CCleaner.