This internal error appears to be related to the number of fields that can be defined to an individual data structure being exceeded.
If the number of field definitions inside a data struture (record) is an excessively large number - the NAT6235 is produced.
In the case we have seen, a file had been created by a Natural program that took one ADABAS record and wrote it out as a single record with all recurring fields concatenated (and there were a large number of recurring fields with large numbers of occurrences).
A second program then attempted to read this sequential file, but rather then having the concatenated fields treated as arrays; they were treated as individual elements - a WHOLE BUNCH of individual elements. When this second program was executed - the NAT5235 would result.
Changing the second program to read arrays for groups of like elements corrected the issue.
General testing indicated that the upper limit was somewhere around 600+ individual fields.