tile_storage_hook: Missing tile was not rendered in time. Returning File Not Found

Hello,
I have a osm_tiles server running in ubuntu16 with Mapnik 3.0, Mod_tile and renderd. in the apache's log appears the error:

[Mon Mar 18 10:02:03.415398 2019] [tile:debug] [pid 3467] ./src/mod_tile.c(325): [client 10.10.21.80:36776] get_storage_backend: Retrieving storage back end for tile layer 0 in pool 7fdfd430d028 and thread 140599315302272
[Mon Mar 18 10:02:03.415404 2019] [tile:debug] [pid 3467] ./src/mod_tile.c(338): [client 10.10.21.80:36776] get_storage_backend: No storage backends for this lifecycle 7fdfd430d028, creating it in thread 140599315302272
[Mon Mar 18 10:02:03.415409 2019] [tile:debug] [pid 3467] ./src/mod_tile.c(354): [client 10.10.21.80:36776] get_storage_backend: No storage backend in current lifecycle 7fdfd430d028 in thread 140599315302272 for current tile layer 0
debug: init_storage_backend: initialising file storage backend at: /var/lib/mod_tile
[Mon Mar 18 10:02:03.415438 2019] [tile:debug] [pid 3467] ./src/mod_tile.c(1391): [client 10.10.21.80:36776] tile_translate: op(tile_serve) xml(default) mime(image/png) z(14) x(11705) y(6833)
[Mon Mar 18 10:02:03.415444 2019] [tile:info] [pid 3467] [client 10.10.21.80:36776] tile_storage_hook: handler(tile_serve), uri(/osm_tiles/14/11705/6833.png)
[Mon Mar 18 10:02:03.415487 2019] [tile:debug] [pid 3467] ./src/mod_tile.c(375): [client 10.10.21.80:36776] tile_state: determined state of default 11705 6833 14 on store 5632ea052600: Tile size: -1, expired: 1 created: 0
[Mon Mar 18 10:02:03.415496 2019] [tile:debug] [pid 3467] ./src/mod_tile.c(176): [client 10.10.21.80:36776] Connecting to renderd on Unix socket /var/run/renderd/renderd.sock
[Mon Mar 18 10:02:03.415525 2019] [tile:info] [pid 3467] [client 10.10.21.80:36776] Requesting style(default) z(14) x(11705) y(6833) from renderer with priority 5
[Mon Mar 18 10:02:03.416967 2019] [tile:debug] [pid 3467] ./src/mod_tile.c(970): [client 10.10.21.80:36776] tile_storage_hook: Missing tile was not rendered in time. Returning File Not Found

What it means and how I could fix it?
Thanks
Vinod

Start by tracing tile_storage_hook and the related paths in ./src/mod_tile.c, using the logged request for z14/x11705/y6833 as a reproduction case. Inspect the renderd Unix socket interaction and determine why the missing tile is not rendered before the request returns. Done means the cause is identified and the request behavior is corrected or documented.