After deleting the original user via SDM and creating a new one (privilege level 15, view name "SDM_Administrator (root)") I'm unable to use "enable" command via telnet any longer.
It asks me for a password, but neither the old nor the new one are working.
What can I do ?
Still looks like your enable secret password or telnet password is holding you out. Also, AAA is working as it should be. You need to go to site where the router is a follow the procedure in the document below. It is comprehensive and accurate. This works for a 1721 also, please read and understand before doing it.
This will allow you to see the config that is stored on the router and then you will be able to decrypt the passwords.