(Workaround found) render_list_geo.pl returns "rendering failed with command 4, pausing", where do I find why?

Have installed openstreetmap-carto on Ubuntu 18.04. Have also imported whole planet into postgresql. Using generate_tiles.py on a part of denmark, generate tiles pretty fast for zoom level 1-13 (about 6 minutes).

When I try to generate for most of the world, first 4 tiles (zoom 1) took about 45 minutes. (When I opened another terminal window and run top (while the generate_tiles.py was running), then I can see that there are many postgresql processes, but most of them are marked with D in the S column, and sometimes R. I do not understand why they are not running most of the time)

UPDATE START I added some indexes to the database, and now the generate_tiles.py is flying, and generating tiles superfast. As much as I would like to get renderd to work, it is not so important anymore since what I need is to pre-render the tiles. Since this now works so fast, I can skip fiddling with renderd and the perl script I mentioned below (for now). UPDATE END

So I just closed the terminal window and installed the script render_list_geo.pl, to see if there was any improvement. This is what I get in the terminal window when I run the script for a part of denmark:

    ./render_list_geo.pl -n 4 -m default -z 7 -Z 7 -x 7.9 -X 10.97 -y 55.39 -Y 57.06

Rendering started at: sø. 16. sep. 19:31:50 +0200 2018

render_list -a -z 7 -Z 7 -x 66 -X 71 -y 39 -Y 47 -n 4 -m default
debug: init_storage_backend: initialising file storage backend at: /var/lib/mod_tile
Rendering client
Starting 4 rendering threads
Rendering all tiles from zoom 7 to zoom 7
Rendering all tiles for zoom 7 from (66, 39) to (71, 47)
Waiting for rendering threads to finish
rendering failed with command 4, pausing.
rendering failed with command 4, pausing.

*****************************************************
*****************************************************
Total for all tiles rendered
Meta tiles rendered: Rendered 2 tiles in 10.00 seconds (0.20 tiles/s)
Total tiles rendered: Rendered 128 tiles in 10.00 seconds (12.80 tiles/s)
Total tiles handled: Rendered 2 tiles in 10.00 seconds (0.20 tiles/s)
*****************************************************
*****************************************************
*****************************************************

Zoom factor: 7 finished at
sø. 16. sep. 19:32:00 +0200 2018

Rendering finished at: sø. 16. sep. 19:32:00 +0200 2018



**Which file or files do I have to inspect to find out what cause this:
"rendering failed with command 4, pausing."**

When I look in the directory: /var/lib/mod_tile
then there is no tiles from the script (render_list_geo.pl).

When I look in the /var/log/syslog I see this, but did not understand much:

    Sep 16 20:08:40 Mapnik renderd[1438]: DEBUG: Got incoming connection, fd 15, number 1
Sep 16 20:08:40 Mapnik renderd[1438]: DEBUG: Got incoming connection, fd 16, number 2
Sep 16 20:08:40 Mapnik renderd[1438]: DEBUG: Got incoming connection, fd 17, number 3
Sep 16 20:08:40 Mapnik renderd[1438]: DEBUG: Got incoming connection, fd 18, number 4
Sep 16 20:08:40 Mapnik render_list: DEBUG: Sending render cmd(6 default 7/66/39) with protocol version 2 to fd 3
Sep 16 20:08:40 Mapnik renderd[1438]: DEBUG: Failed to read cmd on fd 16
Sep 16 20:08:40 Mapnik render_list: DEBUG: Sending render cmd(6 default 7/66/47) with protocol version 2 to fd 6
Sep 16 20:08:40 Mapnik renderd[1438]: DEBUG: Connection 1, fd 16 closed, now 3 left
Sep 16 20:08:40 Mapnik renderd[1438]: DEBUG: Got incoming request with protocol version 2
Sep 16 20:08:40 Mapnik renderd[1438]: DEBUG: Got command RenderBulk fd(15) xml(default), z(7), x(66), y(39), mime(image/png), options()
Sep 16 20:08:40 Mapnik renderd[1438]: DEBUG: Failed to read cmd on fd 17
Sep 16 20:08:40 Mapnik renderd[1438]: DEBUG: Connection 1, fd 17 closed, now 2 left
Sep 16 20:08:40 Mapnik renderd[1438]: Received request for map layer 'default' which failed to load
Sep 16 20:08:40 Mapnik renderd[1438]: DEBUG: Got incoming request with protocol version 2
Sep 16 20:08:40 Mapnik renderd[1438]: DEBUG: Sending render cmd(4 default 7/66/39) with protocol version 2 to fd 15
Sep 16 20:08:40 Mapnik renderd[1438]: DEBUG: Got command RenderBulk fd(18) xml(default), z(7), x(66), y(47), mime(image/png), options()
Sep 16 20:08:40 Mapnik render_list: DEBUG: Got incoming request with protocol version 2
Sep 16 20:08:40 Mapnik renderd[1438]: Received request for map layer 'default' which failed to load
Sep 16 20:08:40 Mapnik renderd[1438]: DEBUG: Sending render cmd(4 default 7/66/47) with protocol version 2 to fd 18
Sep 16 20:08:40 Mapnik render_list: DEBUG: Got incoming request with protocol version 2
Sep 16 20:08:50 Mapnik renderd[1438]: DEBUG: Failed to read cmd on fd 15
Sep 16 20:08:50 Mapnik renderd[1438]: DEBUG: Connection 0, fd 15 closed, now 1 left
Sep 16 20:08:50 Mapnik renderd[1438]: DEBUG: Failed to read cmd on fd 18
Sep 16 20:08:50 Mapnik renderd[1438]: DEBUG: Connection 0, fd 18 closed, now 0 left
Sep 16 20:08:55 Mapnik org.gnome.Shell.desktop[1631]: Window manager warning: Buggy client sent a _NET_ACTIVE_WINDOW message with a timestamp of 0 for 0x3400081 (*syslog (/)