No idea about this mistery: did you check that your Permissions are still correctly set? (just a guess), and that no category or object permissions are interfering with global permissions?
Read the documentation about Permissions carefully, if you haven't done so already.
And you'd better upgrade to Tiki 5.3 at least (or latest from 5.x branch if any older), since some security issues have been fixed.
See Upgrade and ensure you follow the steps to fix the charset issue also to keep having safe and good Tiki usage!