Widgets not saving (again)

In the admin section for allocating widgets to sidebars, when i drag & drop a widget to a sidebar (or footer) and i go to another section and come back or refresh the page, the widget is not saved. The widget do not appear in the public site either.

This happens for a specific site but not for another one with the same WP version and theme. So it seems this is not something related to the navigator (Safari, but i checked with another one also).

I tried changing the theme (back to default for example) but this does not change the widget behavior.

However, at this point, some widgets were in the deactivated widgets list. I tried to drag&drop some of these to a sidebar and then, they were saved correctly. Unfortunately, i cannot put some widget that i choose directly or indirectly (via some sidebar) in the deactivated widgets list. So this is not completely a work around.

I tried removing all plugins, no change.

I looked on forums and saw that it could happen when registering sidebars id which are not lowercase or different from sidebars name. Checked that it was not the case for the theme. I also tried to change some of those things directly in the code. No change.

I tried “resetting the widgets” using some plugin. No change.

I looked in the database. The “sidebars_widgets” element reflects correctly the situation on the widget section. I tried updating manually the “sidebars_widget” with the precise syntax to move a widget to a sidebar. This was not reflected in the widget section.

I’m a bit lost now. Does someone have another idea?

Thanks,
Pascal