<p>My CR-10 S5 has a feature, that stops the print, when the filament runs out.</p>
<p>However, when the printer pauses, the bed cools down and the print plops if the bed. Is there a way to tell the printer to keep the bed heated, when paused (by the runout detector)?</p>

This is varying underextrusion due to loss of material to oozing in the interior of the model.
When printing the infill pattern, the nozzle doesn't follow a single continuous extrusion path, but moves from the end of one path to the beginning of the next, and under Cura defaults,
does this without retracting the filament
. This causes unpredictable amounts to ooze out during travel from one to the next, thereby desynchronizing the planned/intended amount of material extruded so far and the actual amount. This means, when the next outer-wall extrusion starts, there's an unpredictable deficiency between the amount of material at the nozzle to extrude, and the amount the slicer intended to extrude. The result is what you're seeing.
To fix it, you need to eliminate oozing, not just outside the model where it appears as visible stringing, but inside too. Either disable "Combing" entirely in Cura, or set "Max Comb Distance With No Retract" to something very low (0.8 mm or less). Also set "Minimum Extrusion Distance Window" to 0 to ensure Cura doesn't skip retractions for other reasons.
You may also want to play with extrusion length and speed. Too short or too long can be bad; 5-7 mm is the reasonable range for PLA with a bowden. Higher speed generally helps too; the printer should be able to handle 50 mm/s or faster.