PANIC: could not write to log file

After upgrading a pretty big database (serveral hundred gig) from 8.2 to 9.2 I'm getting some "PANIC:  could not
writeto log file" messages. Actually I got two of them. One yesterday and one today. 
  * postgres 9.2.1 is running on a windows 2003 server (downloaded the zip archive binaries)  * Data is on an iscsi san
(thesame postgres 8.2 was running on perfectly since several years)  * Full messages are : 

PANIC:  could not write to log file 118, segment 74 at offset 12189696, length 475136: Invalid argument
STATEMENT:  COMMIT

PANIC:  could not write to log file 117, segment 117 at offset 5660672, length 4096000: Invalid argument
STATEMENT:  COMMIT
  * I have no other messages from the san nor from windows
  * Here are the non default parameters in postgresql.conf

listen_addresses = '*'      # what IP address(es) to listen on;
max_connections = 200         # (change requires restart)
shared_buffers = 320MB         # min 128kB
work_mem = 50MB            # min 64kB
maintenance_work_mem = 700MB      # min 1MB
checkpoint_segments = 15      # in logfile segments, min 1, 16MB each

  Any idea on what might be going on ?
  Cyril VELTER