This error occurs in SDE 3.x for Informix when trying to create a new layer:
Error: Underlying DBMS error (-51).
Error: Cannot Create Layer.
[Informix][Odbc Informix Driver]Data truncated.
The SDE metadata tables only allow table names of 18 characters or less.
This is a known limitation that will be fixed in a later release. In the meantime, use layer names of 18 characters or less.