fabi.b, in your case, php wp_login_form();
is invalid PHP code.
Try this on a separate PHP web page:
<?php
php wp_login_form();
?>
Your error log will give you a better error message than Insert PHP will.
Actually, just
<?php
wp_login_form();
?>
(without the “php” part of the line) most likely won’t work, either.
Insert PHP doesn’t have access to variables and such outside its own code block. See
https://www.willmaster.com/software/WPplugins/insert-php-wordpress-plugin-instructions.php#functionscope
for more info about that.
The above “separate PHP web page” technique is a very good way to test the code before using it in Insert PHP. You get better error messages and, once it passes on your separate PHP web page, it should work just fine in Insert PHP.
Will