Renderd doesn't update tiles

I am running a tile server on an Ubuntu-based virtual machine and configured it using the guide the guide on switch2osm.org. It's just for private purposes, so it's not public. In the beginning, everything was just fine and I could import large amounts of map data without problems. Changes became visible soon after. I have done some minor edits to Mapnik's osm.xml file since then, changing things such as water color or label font size. Since a couple of days, my tiles don't seem to get updated anymore.

When I use

render_list -f -a

this normally gives me no errors. Well, actually that means it goes until

Rendering all tiles for zoom 6 from (0,0) to (63,63)

and then it gets stuck forever. Sometimes it doesn't get that far and gives me an error message saying

rendering failed with command 4, pausing

I can restart Renderd just fine and get no errors. I also often use

touch /var/lib/mod_tile/planet-import-complete

to notify it of any change.

Why are my tiles not getting updated anymore? I have my machine switched on for more than 12 hours and yet I see no changes on my slippymap.

Did my edits to Mapnik's osm.xml maybe corrupt the file and therefore make updating tiles impossible for renderd? Is my machine too slow and it takes weeks to render the whole planet? (I have 16 GB RAM and an 8 core CPU) What else could be the reason?