If you attempt to create a summation with the upper bound of sec(x) and a non-zero addend, Desmos will break and refuse to load anything - equations will cut off if you scroll out, and if you try and type a new equation it won’t render anything and will give you a warning. I believe this occurs with any lower bound.
Sums with arbitrarily high upper bounds can take arbitrarily long to compute. sec(x) has singularities where it grows to infinity. If you ask the calculator to do something that takes forever, it will stay busy trying to do that until you reload the page, and it won’t be able to answer any more questions.
Unfortunately we don’t give great indications that the calculator is busy working, but thats something we could look into in the future.
Suggested calculator warning: “just a minute counting to infinity”