Hi @rodicaelena,
I removed all custom plugins and still got the same result.
With some further digging, I found the cause to be line 69 in the Admin.php
file from the theme.
Currently it is:
add_action( 'activated_plugin', 'after_otter_activation' );
So I changed it to:
add_action( 'activated_plugin', array( $this, 'after_otter_activation' ) );
And now the error is gone.
I’m no WordPress or PHP expert so maybe this isn’t the correct solution, but this line was the cause of my issue.