the_content() isn’t showing content, but $post->post_content does

After the <ul> part you do reset the query, but you also have to reset the original postdata:

wp_reset_postdata();

and in the line

<li>
<a href="https://wordpress.stackexchange.com/questions/77494/<?php the_permalink(); ?>" <?php if (!isset($_GET['tab'])) {echo "class=com_sel"; $page = "אודות הקבוצה";} ?>>אודות הקבוצה
</a>
</li>

the php and if tags are mixed up, i don’t know if PHP understands it like that.

should be like this:

<li>
<a href="https://wordpress.stackexchange.com/questions/77494/<?php the_permalink(); ?>" <?php if (!isset($_GET['tab'])) {echo "class=com_sel"; $page = "אודות הקבוצה";} ?>
        אודות הקבוצה
</a>
</li>

I think this is a mixup with rtl and ltr direction of the language.