When a PC is connected to a switch port, firstly the port issues an 802.1x authentication challenge to the attached device (the 802.1x supplicant). The supplicant replies with username and password and then an authentication request is passed to a configured Radius server.
The error message can look similar to this:
%DOT1X_SWITCH-5-ERR_ADDING_ADDRESS: Unable to add address [mac address] on Fa0/10
This means that the supplicant MAC address cannot be added. This can happen when the TCAM is full or the address exists as a secure address on another port.
The message can come from the port-security subsystem. In some cases, these messages are generated by switches that run Cisco IOS Software Release 12.2(25)SEE2.
If this message appears only once, then this can be ignored.
In order to resolve this issue, if the hardware is full, remove some Dynamic addresses with the clear mac address-table command, which are automatically added to the address table and dropped from it when they are not in use.
If the supplicant address is secured on another port, manually remove it from that port with the clear port-security command.