NAT0777 after upgrading to Natural 414

Good day,

We upgraded our natural production environment from 3.1.6 to 4.1.4 this past weekend. We still running Adabas 7.1.3.
Everything seemed fine until we started getting the NAT0777 buffer pool full in the cics log two days after the upgrade.
We then upped the NTBPI size from the default of 256 to 500. This does not seem to have resolved the problem as we still get the errors intermittently.
Has anyone experienced this? It is really getting frustrating.

Your help will be much appreciated.