IOSTAT 30 problems

Is it possible to analyze the cause of IOSTAT=30 errors from an open statement?

We have

          inquire(file=zfile,exist=file_exists,opened=file_open, number=u,&    action=action)
          OPEN(NEWUNIT=IUNIT, ACCESS='SEQUENTIAL',ERR=999,FILE=ZFILE,&        FORM='FORMATTED',IOSTAT=IOSTAT,STATUS='REPLACE',iomsg=msg)

which gets executed many times, with the parameters in the inquire statement always being returned as .true.,.false.,-1,'UNDEFINED'

However at the end of an analysis step when the routine containing this code is executed the iostat error is set to 30. Subsequent calls to open other files in the same directory then fail in the same way. The iomsg just says 'open failure' (along with the file name and unit number) so that isn't very helpful. I'm fairly sure the problem must be in the code called prior to this but I've no idea how to proceed. Is there a way of finding out what the particular problem is that has caused this error?

Thanks.