Redirect loop, add-{type}-node redirects to add-{type}-node for non-admin users when parent type not availab.e
The Rules module that can be used to perform redirects also. So check if your sites uses this module. If so, verify if you have any "Active" Rules (and/or Rules Components) with a Rules Action that issues a redirect.

If no such Rules can be found, then it is not Rules causing your issue. If such Rules (and/or Rules Components) do exist, then temporary disable all such Rules and check if the redirect loop still happens.

If the redirect loop no longer happens, then re-enable the temporary disabled Rules again, one by one, until the problem (loop) starts again. Then disable that Rule again and use Rules debugging facilities to find out what the (customization related) problem with that rule is. For more details about debugging facilities in Rules, refer to my answer to ""How to loop through comments using the Rules module?".