Ubuntu – strange conky background behavior

I have created a custom conky showing weather forecasts in 3 cities like this:

But recently I've begun to see a strange behavior. After a few seconds, a white background a bit larger appears underneath like this:

I've attempted to overcome this by trying a number of changes in the configuration file .conkyrc and the only effective one was changing own_window_type value from override to normal. But then I not only have this conky disappear when I choose to minimize all windows, but it also displays a transparent background of the same size like this:

My conky window settings are like this:

update_interval 1
total_run_times 0
cpu_avg_samples 1
net_avg_samples 1
no_buffers yes
double_buffer yes
imlib_cache_size 0

background yes
own_window yes
own_window_type override
own_window_transparent yes
own_window_hints undecorated,below,sticky,skip_taskbar,skip_pager
minimum_size 300 480
maximum_width 300
alignment top_right
gap_x 2
gap_y 28

draw_borders no
draw_graph_borders no
stippled_borders 0
border_inner_margin 0
border_outer_margin 0
border_width 0

text_buffer_size 1024
override_utf8_locale yes
use_xft yes
xftfont Ubuntu:size=10
xftalpha 1.0
uppercase no
temperature_unit celsius
draw_shades no
draw_outline no
default_color C0C0C0
default_shade_color 000000
default_outline_color C0C0C0
color1 C0C0C0
color2 B2B2B2
color3 808080
color4 D4D4D4

TEXT
${image ~/.conky/base.png -p 0,0 -s 300x480}

Can somebody tell what might be causing (and how can I overcome) this unwanted canvas underneath my conky window?