[Nando_e90]

Could be the ballast. You can first try swapping the ballasts, the one on the right headlight to the left and viceversa.
Had similar symptoms when mine went bad. Sometimes the passenger side xenon worked and sometimes it went out. The ballast was the cause.

If not, you will have to check the inside connections as stated above, but take into account that the wiring isolation could be already cristalized and with little movement it starts to fall apart. You will need to rebuilt the isolation if so. Happened to me too lol.