It’s because plugin path is not default.
Solution: in file /assets/css/pum-admin-bar.css
You must change 2 lines (7 and 15)
background: url(/wp-content/plugins/popup-maker/assets/images/admin/icon-info-21x21.png) center center no-repeat transparent !important;
to:
background: url(../images/admin/icon-info-21x21.png) center center no-repeat transparent !important;
and
background-image: url(/wp-content/plugins/popup-maker/assets/images/admin/icon-info-21x21.png) !important;
to
background-image: url(../images/admin/icon-info-21x21.png) !important;
