cannot perform this operation. the file has been deleted or someone else has it checked out

Good morning,

I have a problem with my Sharepoint 2013. when I try to open a document appears the error: can not perform this operation. the file has been deleted or someone else has it checked out. But what catches my attention is that this problem only occurs when I enter the site through a public IP, but when I do it from the internal network is not problem.

Any Ideas???

PD: Excuse my English, but I speak Spanish :)