When trying to close it, pay attention to the text - if it changes every time you press the x button, you're actually closing it, there's just hundreds of them (that should be the case, actually)
To find out what's causing it, turn off your addons and turn them back on one by one until you get the error again.
Relthion: CP810 DK Tank - vMOL HM, vHOF HM, vAS HM, vCR +2
Malorson: CP810 Mag Sorc - vMOL HM, vHOF, vAS HM