AF when instance starts because plog overflow

Hello I have a 11.70FC4 instance that fails to start because of physical log shortage, here is the extract from online.log : 04:44:38 Logical Recovery has reached the transaction cleanup phase. 04:44:38 Performance Advisory: The physical log is running out of room during checkpoint processing. 04:44:38 Results: Transactions are being blocked until the checkpoint is complete. 04:44:38 Action: Increase the physical log size. 04:44:38 Assert Failed: Physical log file overflow 04:44:38 IBM Informix Dynamic Server Version 11.70.FC4 04:44:38 Who: Session(1, informix@afb02, 0, 700000361f00028) Thread(7, main_loop(), 700000361ebe028, 1) File: rsbuff.c Line: 4016 04:44:38 Results: Dynamic Server must abort 04:44:38 Action: Reinitialize shared memory 04:44:38 stack trace for pid 15007744 written to /appli/informix/IDS11.70/tmp/af.3efa5a4 04:44:38 See Also: /appli/informix/IDS11.70/tmp/af.3efa5a4, shmem.3efa5a4.0 As the instance doesn't start, I can't increase the physical log size... I also have set the PLOG_OVERFLOW_PATH config parameter but it doesnt help, for an unknown reason it seems that this parameter is ignored because I cannot see any new file created inside the directory. I have even tried to start in fast recovery mode using the "oninit -PHY" undocumented command but then I can't run onparams to increase the plog size as the command returns "shared memory not initialized" ... Does anyone have some clue to make this instance start again or do I have only the option to restore from a previous backup ??? Thanks in advance Fabrice