Some years ago I assempled the K4020. It works fine, but there’s one problem which has been there now for about 2 years:

The “click” sound, normally occurring a few seconds after I switch the unit on, occasionally keeps comes back, in one of the channels, at irregular intervals (sometime infrequent, sometimes a number of clicks). The problem is there also if I swap the loudspeakers (and also at very low volumes - no CD being played), so I do believe that this is due to a K4020 component.

I guess that the problem is related to the automatic protection system. RY1 coming on and off, but why? Could somebody please advice me on how to solve this problem?

Hard to tell without actually being able to examine the amp. Furthermore, if it happens occasionally, it will be difficult to troubleshoot.
If I remember well, we’ve changed zener diodes ZD5 & ZD7 from 39V to 43V, it had something to do with unwanted tripping of the DC protection. You might want to give this a try.