POL 1160: Happens a lot when SE servers are busy. If you're trying to log in right after an update take a break and try again later.
POL 5311 or POL 5312: Seems to happen only to people with authenticator. Usually means you have weirdly linked accounts. Make sure you use the right SE ID and that your POL ID is saved properly.
POL 5426: Most of the time it means a maintenance is going on. Check the FFXI site for any related news.
LM-11 or LM-17: Either a temporary 3 days ban or a permanent ban. In other words you screwed up.