It seems that when I set BCFFTT = PERIODIC, not only the z-direction but all directions are automatically set to periodic boundary conditions. @uldis_l I am assuming "UL" in the comment of PartBunch::setBCForDCBeam() is you. Was there a particular reason to do this? In my understanding, a DC beam would have open BC in x and y and periodic BC in z.
In addition, the manual calls the parameters "BCFFTZ" and "PARFFTZ" but OPAL tells me those don't exist and throws an Exception, I have to use "BCFFTT" and "PARFFTT". Just a minor bug.