SAP Listeners programID not registered CPIC err

Problem in a nut shell:
We are having a very strange problem with one of our SAP listener.
We send multiple documents at the same time from SAP. Some makes it right away. Some throws back errors in SAP:

WEBMETHODS 24.11.2004 11:23:28 program G7P700HOEWISPRD not registered / CPI-C err 837E106B240E41A492071987

No errors in webMethods logs. Then, eventually, the failed documents makes it to webMethods (SAP retries every 15 minutes). It takes a long period or times (multiple retries - hours or delay).

Anybody experienced these symptoms before ?

IS 6.0.1
SAP Adapter 4.6 (with a all service pack)

Thanks,
Jean-Fran├žois