

Blackberries are supposed to have natural pectin in them. You could be straining them too much and removing the natural pectin from the juice. Idk worth experimenting with I’d say
We do add butter to ours to help reduce the amount of foaming during the boiling process. So that might have something to do with it too
You might be able to fix that depending on what the internal setup is. Looks like the top of your tower, shouldn’t be too hard to get to