Sounds like they didn't really disable mod_security at all. There is no other reason why loading that page would block you from the site.
I think perhaps a better solution is to only comment out the <iframe> line (line 986 in current version - see here). That way you still see the options so you can change it.
Edit: OK I tried installing mod_security on a test server and adding the LDAP rule but I am not able to reproduce the issue.
But I've just made a new commit ready for v1.7.2 that I think should fix this. Would you be able to test it for me? Check in qa-base.php at line 273 and change the definition of QA_URL_TEST_STRING to this:
define('QA_URL_TEST_STRING', '$&-_~#%\\@^*()][`\';=:|".{},!<>?# π§½Жש');
And uncomment the code you commented from admin-default.php. Thanks.