In version 2.5, you could add the following to a SafeCracker form:
<input type=“hidden” name=“relationship_field” value=“ENTRYIDOFRELATED” />
This throws PHP errors in EE 2.6 because they expect the field to be an array like this:
<input type=“hidden” name=“relationship_field[data][]” value=“ENTRYIDOFRELATED” />
This is a bug introduced in EE 2.6 that makes forms created with 2.5.x break. The relationship validation should accept either an atomic value or an array.
The solution is to change the syntax of the hidden field as indicated above.