Thanx for the reply. I fixed it.
This is a part of the theme code in quantity-input.php
<input
type="number"
id="<?php echo esc_attr( $input_id ); ?>"
class="input-text qty text"
step="<?php echo esc_attr( $step ); ?>"
min="<?php echo esc_attr( $min_value ); ?>"
max="<?php echo esc_attr( 0 < $max_value ? $max_value : '' ); ?>"
name="<?php echo esc_attr( $input_name ); ?>"
value="<?php echo esc_attr( $input_value ); ?>"
title="<?php echo esc_attr_x( 'Qty', 'Product quantity input tooltip', 'woocommerce' ); ?>"
size="4"
pattern="<?php echo esc_attr( $pattern ); ?>"
inputmode="<?php echo esc_attr( $inputmode ); ?>"
<?php if ( ! empty( $labelledby ) ) { ?>
aria-labelledby="<?php echo esc_attr( $labelledby ); ?>" />
<?php } ?>
<input type="button" value="+" class="plus" />
But if the $labelledby is empty the <input> tag will not be closed.
I changed the last part to:
<?php if ( ! empty( $labelledby ) ) { ?>
aria-labelledby="<?php echo esc_attr( $labelledby ); ?>" />
<?php } else { ?>
/>
<?php } ?>
<input type="button" value="+" class="plus" />
Now it works again!