With version 5.3.5 i have the same problem:
Warning: Illegal string offset ‘acf_fc_layout’ in C:\MAMP\htdocs\pulkus2.dev\wp-content\plugins\advanced-custom-fields-pro\pro\fields\flexible-content.php on line 233
it is flexible-content under a tab.
The error shows up in wp-admin when i publish the page.
