My default upr uses SwitchComboFormula in jlb.ufm plus JLB_SwitchFormula, JLB_SwitchCombo, and JLB_MandelbrotSFormula in jlb.ulb. I had a typo in the Mandelbrot,
default = 1"
in a parameter block. When starting up UF, I got a large number of confusing and not helpful error messages, as in the screen shot below.
When the (blank) fractal loaded, I clicked on the top line, SwitchComboFormula, and got the correct error message, pointing to the bad line.