we have an old Tektronix WFM 601A. It works...and works...and works..

After our 5 weeks summer break, we switch the WFM 601A on and it displays " configuration error....please contact Tektronix Service".
A colleague told me, that their is a certain button combination for it, but he cannot remembert it anymore.
We don't find any help in the service manual or in the www
Anyone had an idea?
thanks a lot
young television guy