Failing to connect to nother server

I have written a server module in our application server solution that uses JBoss. My module is sending  transactions to another server X. The module is doing so by connecting to an API (a local dll that is provided by the party that has the server X. To be able to connect to this dll, I have written an own JNI-bridge dll in C. Got it?

My Server module -> my bridge JNI dll -> client dll from X ------------- (network) ----------> server X

Suddenly the "client dll from X" starts to reply fail on all transactions by claiming that server X is down. No network traffic is sent (so it is lying ;-)   ). After a restart of JBoss, it works for awhile, sometimes many hours and sometimes the first transaction goes into fail mode, which means that the rest of the transactions will get the "X is down return code" back.

What could be wrong here? Are there any security things that could lock up the network connection suddenly. That would sort of fit the error code, it can't reach the server.

Could it be a JNI issue?