Resolution:
Adapter can lose connection with Siebel:
--> if the connection is timed out
--> if there is a network problem
--> Siebel server or a component is restarted or shutdown

Adapter will not process request:
--> if the connection is lost
--> if the transaction is timeout from Siebel.

Following are some reasons and corresponding sample errors:
===================================================
Reason:
When adapter machine's network is ipconfig released or just the Object Manager component is stopped on server
Error:
AESEBL-990148 "Siebel GetBusObject error, for Account (The connection pool is empty and no connection could be created.(SBL-JCA-00310)

Reason:
When adapter or Server machine is unplugged from network or ipconfig is released on Server machine
Error:
MRPC Request 39 was abandoned after 18922 ms. Connection: a00019 due to connection shutdown request Connection reset(SBL-JCA-00303

Reason:
When server is restarted
Error:
&ltError>&ltErrorCode&gt1179754</ErrorCode> &ltErrMsg&gtThe task 12582922 has been shutdown</ErrMsg></Error>
&ltError>&ltErrorCode&gt1180678</ErrorCode> &ltErrMsg&gt42*SBL-SMI-01000: The server is shutting down1*013*smimtpool.cpp4*15510*0*0*0*</ErrMsg></Error>

and then

&ltMajor No.&gt256</Major No.>&ltMinor No.&gt8716591</Minor No.>&ltMessage&gtOMRPC Request 23 was abandoned after 0 ms. Connection: c0000a due to connection shutdown request Connection reset(SBL-JCA-00303)</Message>&ltDetailedMessage&gtUnknown&ltDetailedMessage>

Reason:
When session timeout expires
Error:
2009 Aug 28 12:03:06:802 GMT -7 SiebelAdapterConfiguration Error [Application] AESEBL-990148 "Siebel GetBusObject error, for Account (The task 12582937 has either been shutdown or timed-out).

Reason:
When the Server Request Broker component is shutdown or not responding or txtimeout expires
Error:
OMRPC Request 72 on connection 130002b was abandoned after 250 ms because it timed out.(SBL-JCA-00317)

Resolutions
==========
(1) To avoid connection / session timeout the seibel.connmgr.sesstimeout can be set in siebel.properties file. Please refer to LBN1-7VF5T9 for details.

(2) To avoid transaction timeouts the siebel.connmgr.txtimeout can be set in siebel.properties. Following is some information on this:
The following is the description of the above property:
siebel.conmgr.txtimeout
================================================
The number of milliseconds to wait after sending a request to the Siebel Server. Must be a positive integer; other values are ignored. The default is 600000 milliseconds (10 minutes).
=================================================
The properties "siebel.conmgr.txtimeout" and "siebel.conmgr.sesstimeout" need to be added in the siebel.properties file and this file can be placed in the &ltTIBCO_HOME>/adapter/adsbl/5.2/hotfix/lib folder which will be in the adapter's classpath.