Here is same place rendered at same zoom. In left rendering is performed using renderd, in right - using nik2img.py. As you can see there is visible intersection between metatiles due to improper raster scaling around edges. All scaling methods shows +/- same issue.
@ramunasd - yes, this is a known issue. I'll have a look what we can do maybe mirroring pixels at edges might improve visual output.
+1 We are having a similar issue when using tiled raster images, the tiles have 1px borders at some zoom levels where the Map background-color is applied.
Could this be the same problem for adding raster files AND background color?