Mouse Button 4 and Mouse Button 5 are not recognize are are unable to be reassigned. Logitech SetPoint software is set to defaults and Game Detection is turned on.
Ok... i just found a fix
Not sure though why the buttons worked before and then suddenly stopped... the solution is to set the extra mouse buttons in the logitech setpoint software to "Generic button". Now they should work again in the game. There is also the possibility to use this setting only for a specific application like ESO.
Hope that helps. Finally, I have my mouse buttons back!