Unemployment causes unrest. Did your production modidifer drop to 0% (or stabilize at some level where expenses exactly equaled (reduced) income?)
The production modifier did not drop in any of the cases that I saw zero monetary production. And still no unrest. When I have a day off I'll do a fresh install again and see I can recreate this. As the questions you are asking should be relevant, and there should be an indication of some sort to what the cause is. I like a good puzzle, but this is like one of those that bugs you like a gnat flying around your head that you just can't squish.
Here is my testing setup
Latest install
Sol start - known stars Sol start - known stars
pop levels 250; 350; 500; 700 pop levels 250; 350; 500; 700
TN conventional
Sol start - known stars off Sol start - known stars off
pop levels 250; 350; 500; 700 pop levels 250; 350; 500; 700
TN conventional