We are currently doing a sweep of all mature series on the site to look for and remove images that are deemed pornographic.As of now we don't have a list of what these mature series are or their titles but OneManga furter says:
During this time, all mature series will be unavailable for viewing. We will post an update here once the sweep has been completed. Some series will be coming back. Others will not. Hopefully the final outcome will not be too bad.
We are sorry for the inconvenience during this time, but do understand that in the USA, there is very low tolerance for pornographic content on sites like OneManga that attract a sizable crowd of younger readers. If we do not do a sweep like this, OneManga may be closed forever.
Thanks for your support and understanding.
[Update: To clarify, we are not removing all the mature content forever. We are simply removing them until we have time to manually check the content of the series. As series get cleared, we will add them back.]So if you are getting a page error 404, it is most likely because admin has suspended the series in question while they do a sweep for inappropriate content on the site.