Cannot Check In Document After a Document Library Has More Than 15 User-Defined Person Fields [SharePoint]

I am working with a document library in SharePoint 2010 which has 16 user-defined person / group columns (excluding created by, modified by, etc).

But a strange thing starts to happen when I checked in a checked out document. It shows a permission error, but it works when i tried to reduce the person / group columns to 15.

The same thing happened when I try to update a list item with my custom edit form, exception about permission (The document blabla has been modified by blabla), but it works when I reduce the person / group columns to 15.

Is there any limitations from Microsoft in maximum number of person / group in a document library? Because i've researched and found that 96 is the maximum number of person/group column in a list / document library (which means it should be fine in my case).

If it is not about the limit, then what is really happening with my document library?

Any help will be greatly appreciated.

Thanks.