Cannot change the map's background png

[schnudd31do3]

When replacing a background picture in
/usr/local/nagvis/share/userfiles/images/maps
with its same file name, the privious version of this picture is shown. Also if I delete the privious version, it nevertheless is shown.

When I rename the new version of the file and select it in the map's configuration

it then will be shown.

If I then rename the new version with the old name, the old version is shown, even there is only the new version left in the above named folder.

How can I reset its cache or is it a bug?