error after switching URL

Hi, Built site, today pointed domain to server and set up SSL since then I have had these error codes:

The image gallery URLs are all updated to new domain, but none appear in library when your AWB plugin is activated.

Warning: Illegal string offset ‘cache’ in /homepages/5/d400591674/htdocs/flexcleaning/wp-content/plugins/advanced-backgrounds/advanced-backgrounds.php on line 206

Warning: Cannot modify header information – headers already sent by (output started at /homepages/5/d400591674/htdocs/flexcleaning/wp-content/plugins/advanced-backgrounds/advanced-backgrounds.php:206) in /homepages/5/d400591674/htdocs/flexcleaning/wp-includes/functions.php on line 6029

Warning: Cannot modify header information – headers already sent by (output started at /homepages/5/d400591674/htdocs/flexcleaning/wp-content/plugins/advanced-backgrounds/advanced-backgrounds.php:206) in /homepages/5/d400591674/htdocs/flexcleaning/wp-admin/includes/misc.php on line 1252

Warning: Cannot modify header information – headers already sent by (output started at /homepages/5/d400591674/htdocs/flexcleaning/wp-content/plugins/advanced-backgrounds/advanced-backgrounds.php:206) in /homepages/5/d400591674/htdocs/flexcleaning/wp-admin/admin-header.php on line 9

Not sure how to fix this. I have disabled the plugin, renamed it and then put back again… same error. I don’t have a cache plugin running

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