Solved through elgg_register_plugin_hook_handler

:)))

Elgg-Cropper plugin https://elgg.org/plugins/2576663 of @nikos solved it nicely.
Thank you @nikos

@Reuven: Thanks. Good to know that this plugin is useful for Elgg members

It might be that at some point the registering of the ajax view used got lost. If you add

elgg_register_ajax_view('theme_sandbox/components/tabs/ajax');

in the developers_init() function in start.php of the developers plugin it works.

It is OK now.

Thank you