Hi Stefan,
The problem is TEC uses an older version of select2 (3.x) while Meta Box uses version 4. Searching on the Internet shows quite a lot of similar problems and the solution is use only one version of select2.
In this case, I think to make it work again, it's required to not using select2 from Meta Box. You can do that by switch the field type from select_advanced
to normal select
like this:
add_action( 'mb_relationships_init', function () {
MB_Relationships_API::register(
array(
'id' => 'posts_to_pages',
'from' => [
'field' => [
'post_type' => 'post',
'ajax' => false,
'field_type' => 'select',
'query_args' => array(
'posts_per_page' => -1,
),
],
],
'to' => [
'field' => [
'post_type' => 'tribe_events',
'ajax' => false,
'field_type' => 'select',
'query_args' => array(
'posts_per_page' => -1,
),
],
],
)
);
} );