This solved my problem. I deactivated the following redundant plugins:
• Advanced Custom Fields: Flexible Content Field
• Advanced Custom Fields: Repeater Field
These are very old plugins that are no longer offered nor supported. Their functionality in included in Advanced Custom Fields PRO, so they are redundant.
After deactivating the 2 plugins, I updated ACF PRO to 6.3.10 and there is no fatal error. I then tested functionality by editing posts that included flexible content and repeater fields. All works well. Simple solution for me.
