I have faced another kind of Error ID: 8224, which was, however on virtual machine, not caused by TCP chimney issue, so here is the solution.
When running command
I have received error 8224 as shown in picture:
In my previous article the problem was caused by incorrect settings of TCP interface (https://ficility.net/2012/09/06/extending-schema-for-exchange-2013-error-id-8224/), which caused delays in replication and change actually solved the problem. In this case TCP interface was set correctly and I still experienced delays in replication. Here is how to deal with this kind of problem.
Before extending schema domain controllers hosting FSMO roles (at least Schema master) must be accessible and replication should work correctly. In my case there was a problem replicating Schema and Forest zones to secondary DC, which was connected via VPN. I have resolved VPN connection problems and after that I forced replication between DCs. Note, that MS have incorrect syntax on their technet site. You have to use quotations to run command successfully.
To force replication run the following command from elevated command prompt. http://technet.microsoft.com/cs-cz/library/cc742152(v=ws.10).aspx
repadmin /replicate <SOURCE_DC> <DESTINATION_DC> "<DC=domain,DC=local>"
After successful replication it is possible to extend schema.