Missing tiles in Watercolor
Some tiles are missing. For example here: http://maps.stamen.com/watercolor/#15/43.3944/21.8352
If you try to load a tile that doesn't load, for example: http://a.tile.stamen.com/watercolor/14/9183/5997.jpg you get this message: Service Unavailable
A simmilar thing happens with Terrain layer, but not with Ink layer (you can see in attached screenshot)

Yep, I have the same problem. It starts on zoom level 15, everything above renders just fine. Once you zoom in to 15, tiles just won't load (in my case, this: http://maps.stamen.com/watercolor/#15/52.3329/10.8328)

I have the same issue with zoom level 18. "map -> image" (m2i) also fails with levels > 16 and does not show tiles.
http://a.tile.stamen.com/watercolor/18/138297/84727.jpg -> OK http://b.tile.stamen.com/watercolor/18/138298/84727.jpg -> service unavailable http://d.tile.stamen.com/watercolor/18/138300/84727.jpg -> OK
Same goes for https://stamen-tiles-d.a.ssl.fastly.net/watercolor/18/138298/84727.jpg
What is required to provide debug info?
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https://wiki.openstreetmap.org/wiki/Rendering
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https://wiki.openstreetmap.org/wiki/Maperitive
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http://maperitive.net/
- 2.4.3 only retrieves level 17 tiles
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https://wiki.openstreetmap.org/wiki/Tile_servers
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https://wiki.openstreetmap.org/wiki/Stamen
Huh, so it seems the watercolor renderer is proprietary and there is no way to look into it or render tiles myself.
BTW, this is a duplicate, there are many older reports regarding this issue.
The issues are known, map data / tiles are 5 years old, support has ceased, the renderer is closed-source.