Error with custom assoc type definition as assoc creation fails
To reproduce try:
1.) Model two topic types: "Dummy Parent" (Entity) and "Dummy Child" (Text) and set cardinality to "Many".
2.) Create "Custom Value" (Number) topic type and create a new "Custom Assoc Type" (of datatype Composite). Set up "Custom Value" as a child type to "Custom Assoc Type".
3.) Configure the new "Custom Assoc Type" to be the standard assoc type in between "Dummy Parent" and "Dummy Child".
4.) Create an instance of "Dummy Parent" and try adding one or many "Dummy Child" values. The same error as report here should appear.
Note: While trying to reproduce it for you, I broke the dev server as I accidentially modelled an assoc type as a child topic of an assoc type. You'll probably have to wait for tomorrow, until the db on dev is reset.
FYI: @jri