How to fix wall separation in 3D prints (gaps in between wall perimeters)? The print is very solid except for the 4 walls.

From the top, I can slide a paper down to the bottom. This is ONLY between the walls, the rest of the print is solid. The filament is PLA 1.75&nbsp;mm.

But the bottom is solid, no gaps.

I have checked the usual problems on Ultimaker troubleshooting photo gallery, but I can find anything similar.

Any advice to fix this would be very welcome.



Print settings:
Now that print settings are shared we can see that this problem is not related to too fast printing (only 20&nbsp;mm/s) or too low print temperature (210&nbsp;&deg;C should get PLA fluid enough). To explain this, a low temperature and too fast printing cause under-extruded lines.

There are 2 other causes that might be worth investigating:


Under-extrusion. From the top layers one can see that there may be insufficient material printed. Calibration of the extruder helps in this respect.
Inaccurate positioning. This may for instance be caused by loose belts or a mechanical defect.