Illegal String Offset Warning

Hello! I get the following line on top of thumbnail images that use the default image in the “Related Posts” section of my site:

Warning: Illegal String Offset ‘class’ in ….

https://manilarat.montalut.com/2005/11/the-philippine-tourism-highway/

I use the Jetpack plugin for the Related Posts.

Hope you can help. The plug in works perfectly otherwise.

Thank you!

https://www.ads-software.com/plugins/default-featured-image/