How do I measure the characteristics of a voltage divider in a QUCS simulation?
Like SPICE simulators there's a difference between simulation parameters and reference designators.

Here, the reference designator for the lower resistor is R2. The R value of R2 is always 50 ohms. To use a parameter, you must assign it to the parameter name surrounded by curly braces. So the R value of resistor R2 should be {R2} to have resistor R2's R value set to the parameter R2.

To make things a bit more clear, change the parameter sweep param to RLOW and the value of R2 to {RLOW}. The simulation should work as expected and you'll get the correct results (yeah, I realize that my voltage source isn't at the same level as the OP so the actual values will be different).