we found the nasty 2115 error in the Baan logfiles (log.oracle8):
ORA-01115: IO error reading block from file 21 (block # 76828)
ORA-01110: data file 21: '/oracle/data/baan/dat1/dat5m01.dbf'
ORA-27091: skgfqio: unable to queue I/O
ORA-27072: skgfdisp: I/O error
IBM AIX RISC System/6000 Error: 5: I/O error

In the alert.log of oracle I didn't see this error.
Why is that error not reported on oracle level?

Not all Oracle errors are reported at Oracle alert file. There are a bunch more directories and files that contain Oracle errors. I suggest to investigate the udump, bdump directories on this error.

Hope this helps,
Dick