SmartMag navigation menu not showing anymore

Hi,

my site is not showing the navigation menu anymore.

I’m on SmartMag theme, not the most up-to-date version (before updating the theme I need to tidy up quite a lot of files) but the theme has always worked perfectly, plus this issue I’m having is not coming after a change in the theme nor after a WP update.

Additionally to the navigation menu not showing, I’m getting this Warning

Warning: Illegal string offset ‘custom_css_post_id’ in /home/content/n3pnexwpnas01_data02/84/3490184/html/wp-includes/theme.php on line 905

Of course I haven’t edited the files in wp-includes, and also I think I shouldn’t, the problem must stand in the menu declaration of the theme, even if it’s really weird that the issue appeared all of a sudden.

Already tried to disable all plugins, the issue remains.

Do you have any tips for me? This issue is really annoying me.

Thanks,
Manu