Hi @HCF and @khuslid,
With you original expression:
[SortColumn] must belong to the Activities table, this is the condition for using ORDERBY() (in my opinion).
What I need to do, is to sort the Activities on the [SortColumn], but since this column refer to a column in another table (Activities and not Contacts), the column is not available in the expression builder
I think you pointed it out correctly: you are having trouble because you refer to a column, which refers to another table.
You may want to try this workaround : please create a virtual column in your Activities Table, which is a kind of copy of the [SortColumn], this way:
[SortColumn].[labelColumn] (or any column you think is useful for your sorting)
and change your original expression with:
Does it make a change ?
EDIT : change made with the workaround