Slit signal loss.

When one of the slit signals is gone check the slit impedance.

We ones had an shorted slit. (Several Ohms)

Do not read the next lines if you are an Elekta engineer!!

Take a powerful battery for example one out of a UPS.

Connect it to the shorted slit. Be careful you don't want to melt the centre core of the coax.

When lucky the impedance is high again. (In our case this trick worked. Machine is working fine fore more than 3 months now.)