Rare occurrences of exception xpress.InterfaceError: Incorrect control name

Could it be that in those 2% of the cases in which setting miprelstop fails the license is not available? In order to set a control without having an xpress.problem instance a license is required since otherwise the Xpress API cannot check whether the control you are setting actually exists.

Since you are modifying controls before creating an xpress.problem instance, it is indeed recommended that you explicitly call xprs.init() before doing that (it is recommended to do this using a context manager, see the documentation of that function).

In general, is it important that you set these controls globally? Unless you are creating many xpress.problem instances in different places and all instances need this setting, I think it would be better to change the control values on the xpress.problem instances:

with xp.problem() as p:
    p.controls.miprelstop = 0.01