pre_get_posts with tax_query causes empty result

I need to adjust the main loop for all archive pages in the following way.
I have registered a custom taxonomy “myposttypes”. Every single post is assigned to exactly one term of “myposttypes”.

If it is a category or tag archive page, only posts with “myposttypes->term1” should be included into the query result.

If it is a custom taxonomy archive page only posts with “myposttypes->term2” should be included into the query result.

I’ve already implemented the corresponsing pre_get_posts hook but I’m not getting any results at all for all my archive pages. I have plenty of posts with corresponding taxonomies in my blog instance so there should be results.

Here is my approach:

function wpmw_adjust_archive_queries( $query ) {
    if ( ! is_admin() && is_archive() && $query->is_main_query() ) {
        if ( is_category() || is_tag() ) $term_slug  = 'term1';
        if ( is_tax( 'beitragsarten', 'news' ) ) $term_slug = 'term2';

        $taxquery = array(
            'tax_query' => array(
                array(
                    'taxonomy' => 'myposttypes',
                    'field'    => 'slug',
                    'terms'    => $term_slug
                ),
            ),
        );

        $query->set( 'tax_query', $taxquery );
    }
}

add_action( 'pre_get_posts', 'wpmw_adjust_archive_queries' );

UPDATE:
Here is my adjusted and simplified approach, that still does not work unfortunately. I get an empty result. I var_dump’ed $query and tax_query seems to be added correctly to query_vars.

function wpmw_adjust_archive_queries( $query ) {
    if ( ! is_admin() && $query->is_archive() && $query->is_main_query() ) {

        $tax_query = $query->get( 'tax_query' ) ? : array();

        $tax_query[] = array(
            'taxonomy' => 'myposttypes',
            'field'    => 'slug',
            'terms'    => array( 'term1' ),
            'operator' => 'IN'
        );

        $query->set( 'tax_query', $tax_query );
    }
}

add_action( 'pre_get_posts', 'wpmw_adjust_archive_queries' );

Here is the corresponding part of $query

[tax_query] => Array
                (
                    [0] => Array
                        (
                            [taxonomy] => myposttypes
                            [field] => slug
                            [terms] => Array
                                (
                                    [0] => term1
                                )

                            [operator] => IN
                        )

                )

You use tax_query incorrectly. Take a look at Codex Page

tax_query should be an array which can contain:

• relation – it should be string (AND/OR)
• taxonomy term – array with defined taxonomy, field, terms, and so on.

In your code your setting tax_query to:

$taxquery = array(
    'post_type' => 'post',
    'tax_query' => array(
        array(
            'taxonomy' => 'myposttypes',
            'field'    => 'slug',
            'terms'    => $term_slug
        ),
     ),
);

And it should be:

$taxquery = array(
    array(
        'taxonomy' => 'myposttypes',
        'field'    => 'slug',
        'terms'    => $term_slug
    ),
);

post_type is another param, so you can’t set it inside tax_query.

But… You still should be careful, if you change tax_query – it may have a value already. So if you want to be respectful of that, you should use something like this:

$tax_query = $query->get( 'tax_query' );
if ( ! is_array( $tax_query ) ) {
    $tax_query = array();
}
$taxquery[] = array(
    'taxonomy' => 'myposttypes',
    'field'    => 'slug',
    'terms'    => $term_slug
);
$query->set( 'tax_query', $taxquery );

This way you will add your tax query to already existing queries and not override them.