Not sure if the set up of IPv6 has anything to do with this (most likely not). For the third time now my password (changed during factory reset) works for a few days and then I get "Invalid user name or password. Please try again." The only way I can log in now is to do a factory reset. I've done this twice and here I am again locked out. This is very strange. Please let me know if there's anything I can do.
maybe someone unknown on your network is accessing this switch?
I should start by moving away from username cisco and setting a strong password.
other option is: configure a syslog server and store any configuration changes
After doing a factory reset and configuring user id and password are you sure that you have saved the configuration? I am wondering about the possibility that the switch rebooted for some reason (issue with power, etc) and has lost your user id and password.
What kind of switch is this? When you experience the problem logging in does the switch still work? Is the switch connected to some upstream device (router or other switch)? If so are devices connected on this switch able to communicate with the upstream device? Do you have devices in more than one vlan? And if so are connected devices in different vlans able to communicate?