After digging around some more, we've managed to locate an error. It is still a strange one, however.
The setup is as follows: we have a filter that impacts data generation for two DT tables. Both tables have separate renderDT() & DTOutput() couplings. In principle, they should be independent from each other, as the only common ancestor that they have is the global data frame, plus, of course, the filter itself. Now, the filter works fine as long as the number of rows in each table is greater than 0. However, if even one of the tables has zero rows, BOTH tables are not updated (or not rendered, when this is upon initialization).