NAV 2016, error: Server page is already open

Hi, I encountered interesting problem. I am currently working on migration from NAV 2009 classic to NAV 2016, I have bunch of forms converted via MS
TIF tool to pages.
One page is standard document - page and subpage with lines. But i see this error:


When I try page preview from Dev Env, page looks OK to me:


standalone subpage is running correctly.
I tried to google this error but no results.

Please, can anybody help and point me to correct direction?