Hello @cotecimpianti:
Be sure to update Popup Maker to the latest version, 1.7.11. Per the Changelog in the readme.txt file:
== Changelog ==
= v1.7.11 – 03/14/2018 =
* Fix: Bug where Middle Center option wouldn’t stay selected after saving.
* Fix: Bug with incorrect field dependency for custom height & scrollable options.
= v1.7.10 – 03/14/2018 =
* Improvement: Further improved compatibility with shortcodes that echo/print rather than return content.
* Fix: Bug where cookies wouldn’t always be set in Edge & Safari due to cookie path including the root url.
* Fix: Bug that changed the default tag for popup_trigger & popup_close shortcodes.
* Fix: Bug where extra close buttons didn’t always work correctly.
* Fix: Removal of deprecated function that triggered warnings in PHP 7.2.
Your issue was addressed. in 1.7.10.