if the fan works, the oscilloscope has 45 days...
I don't understand this statement.
Is the fan working; yes or no?
This is what I'm thinking:
If the fan is not working, the scope will overheat, and some part inside the scope will change tolerance.
If the probes are compensated when the scope is overheated, the trace will look good as long as the scope is hot.
When the scope cools off, the part inside the scope will go back to the correct tolerance, and the trace will show the probes are uncompensated.
As the scope heats up again (overheats) the trace will then become correctly compensated.
This seems to describe your problem.
So check to see if the fan is working.