Invalid localization server address specified. Make sure you specified the right server address.

It may be a problem if you have no internet connection. However drupal.org was recently migrated to https en-masse, so that may also be an issue (with the redirection happening improperly possibly). Did you try with https in the setting?

There may have been some problems on localize.drupal.org side at the time you hit this issue, is this still the case?

Same problem.

Can you verify that you have exactly http://localize.drupal.org (without trailing slash).
Can you provide the output of drush ev "var_dump(xmlrpc('http://localize.drupal.org/xmlrpc.php', array('l10n.server.test' => array('2.0'))));"

I have the same error.
drush ev "var_dump(xmlrpc('http://localize.drupal.org/xmlrpc.php', array('l10n.server.test' => array('2.0'))));":

array(5) {
  ["service"]=>
  string(51) "Localization community remote translation interface"
  ["version"]=>
  string(3) "2.0"
  ["supported"]=>
  bool(true)
  ["languages"]=>
  string(487) "prs, af, sq, am, ar, hy, as, ast, az, ms, eu, be, bn, bs, br, bg, my, ca, zh-hans, zh-hant, hr, cs, da, nl, dz, en-gb, eo, et, fo, fil, fi, fr, fy, gl, ka, de, el, gu, ht, he, hi, zh-hk, hu, is, id, ga, it, ja, jv, kn, kk, km, rw, ko, ku, ky, lo, lv, lt, xx-lolspeak, lb, mk, mg, ml, mt, mi, mr, mfe, mn, ne, se, nb, nn, oc, or, os, ps, fa, pl, pt-br, pt, pt-pt, pa, ro, ru, sco, gd, sr, sd, si, sk, sl, es, sw, sv, gsw-berne, ta, ta-lk, te, test, th, bo, ti, tr, tyv, ug, uk, ur, vi, cy"
  ["name"]=>
  string(12) "Translations"
}

Worked for me only hard set: drush vset l10n_client_use_server 1

But on send string through l10n_client I have an error:
php_network_getaddresses: getaddrinfo failed: Name or service not known.

I'm add nameserver 8.8.8.8 on my /etc/resolv.conf and solve my problem! Thanks all! :)

Upgraded to 7.24 and faced the same issue.

@Sahin: probably related to #2124931: Update l.d.o syncing for d.o update of project data.

I have same problem. Hmm....

It's work!!! :) I set "openssl" of PHP

Great, drumm updated the rules so the staging server should be accessible. Lets have some other community members try it and if so we can mark as fixed.

Am I doing something wrong or not this part shall be tested?

After saving the server's address as passing auth tokens via URL (https://drupal:drupal@localize-7.staging.devdrupal.org/) and user's key saved in their profile, still can not upload a new translation to server.

@Balu Ertl: are you a member of the language team you are trying to suggest strings for and are you able to suggest to that team on the l.d.o staging site UI?

@Balu Ertl, @japerry: I opened #2543440: Localization remote submission fails for the server bug, let's leave this issue open because people submitted it regardless of a staging server (vs. the invalid server), so the issue is relevant either way, but the staging problem is not relevant here anymore. Let's continue there.

@Gábor Hojtsy: yep, on the Simplytest site I tried to submit translation to Hungarian locale, and I assigned the key of my group admin account on Staging site. However for counter-testing today I also set up a non-admin simple user both on Simplytest and Staging, but the results are same.

Seems that the issue was solved way back then