Thanks for the extensive feedback, Julius!
I'll check SMGW and see whether I can trace there but will give ST01 another go with all app-servers beforehand (didn't know it was app-server specific 
).
Indeed I forgot to append the system information.. -_-: ECC 6.06 NW AS 7.03 with ABAP Stack 731 Level 6, SAP_HR 604 Level 61, EA_HR 607 (HR_RENEWAL 1) Level 12. Kernel 721 Level 40 compiled for SunOS
@ Andrii, methinks this isn't the root-cause because these authority objects already are assigned and since they are not part of SAP_ALL/SAP_NEW but SAP_ALL/SAP_NEW does do the trick here I wager they are not 'at faut' ;-/
Cheers, Lukas