Hi @Nevv
With the permit 38a you have to seek approval for every job you wish to do, including a minijob. You are also obliged to inform your current employer about a second job. They cannot, however, prevent you from having a second job except in certain specific cases. This article gives a good overview of what to be aware of when taking up a second job (article in English via online translation):
https://***-personio-de.translate.goog/hr-lexikon/nebenbeschaeftigung/?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp&_x_tr_hist=true
Best,
Éanna