@geranis74,
Did it work in the meantime? 🤔 & can you again login into your myCloud account? 😀
If your still logged in on one device you have the possibility to check the username of the swisscom login with which your myCloud is linked by opening the Settings/Account screen.
There under your first- and lastname you can see the username of your Swisscom login with which your myCloud is linked.
Otherwise you can find out the username of the swisscom login with which your myCloud is linked as mentioned by cymett over this site:
If u forgot your password for the corresponding swisscom login have a look here:
or get directly in contact with the regular swisscom support (0800 800 800 or via web here: Contact Swisscom) to reset the password.