Unable to persist global user rights

Testing a 7.3 clean install:

I'm unable to keep rights set, as if I leave Rights page and then come back again all user/group rights I've tuned up remain unset (reseted to "default" I think...). However, unregistered user rights (the ones below group table rights) are working fine!

I've been playing with browser console and I can't see any error in client side. I think request is reaching server as I can read the following both in Network tab in Firefox and in localhost_access_log when I check any of the group rights:
/GET /xwiki/bin/view/XWiki/XWikiPreferences?xpage=saverights&clsname=XWiki.XWikiGlobalRights&fullname=XWiki.TestGroup&uorg=groups&form_token=F1Tx3ax0rIGsf8noimHySQ&action=allow&right=view HTTP/1.1" 200 7/

Note that I can see a different behavior for unregistered user rights: POST instead of GET. As follows:
/POST /xwiki/bin/save/XWiki/XWikiPreferences?form_token=F1Tx3ax0rIGsf8noimHySQ HTTP/1.1" 204 -/