What is the difference between a “complicated” problem and a “complex” one?

Complicated means something has many parts and is hard to understand, but it can be solved with rules, logic, and expert steps.

Complex means something has many interconnected parts that change each other, making the final outcome unpredictable.

See https://www.sfu.ca/complex-systems-frameworks/applications/complex-and-complicated-are-not-the-same.html for a little more information.