It seems that I have solved my DC events, that triggered the e12 from time to time.
I tried to see what happened if using only one channel. So I removed the tube of one channel at a time. I tried all permutations of channel tube removed and tube. As a conclusion: one channel at a time was always working. But two channels at the same time generated those DC events. It could comme from a load problem on the power supplies.
So I went again through the initial setup sequence to check the voltages and idle currents. Not changed a lot, but had to adjust slightly the tail current of the input staage of one channel.
And now seems to work wothout DC events. Let's see over what happens the weeks...