Found this potential solution to your boss problems on reddit:

Mac: Hello welcome please sign in

Dev: Fair enough

Mac: Oh you haven’t signed in in awhile please get get verification from other device

Dev: kk

Mac: Oh you don’t have a dev account, please sign in on this website

Dev: Hm.

Mac: In order to sign up for a dev account you need to download this app

Dev: ???

Mac: Are you sure you want to open this app you just downloaded?

Dev: Sigh.

Mac: In order to sign up for a dev account on this app you need to sign into it

Dev: For the love of god

Mac: Ok now you can build with Xcode.

Xcode: No you can’t. You have to sign in

Dev: fuck sakes.

Mac: Are you sure you want Xcode to access files on your computer?

Dev: …Yup

Xcode: Signing in isn’t enough you have to select the fact you are signed in a dropdown nested 3 menus deep.

Dev: God damn.

Xcode: Build failed please sign in to phone as well.

Phone: New sign in detected, please verify with alternative device.

Dev: Jesus.

Xcode: Build success! Pushing to iPhone.

Dev: Finally.

Xcode: Unknown error occurred. Please go to support.apple.com for help. :)

Dev: …

Since when does the garbage collector running actually increase the memory used??!!!

4 hours wasted due to a rather strange "scope poisoning" bug... it was really fucking strange.
Had to move the 5 lines of code, above where the error happened, to a private function.
The 5 lines just called an XLSX library which read a given file and returned an array. I don't know how but the thing somehow messed with my Soap client call one line below. The culprit must have been some IO resource handle which didn't get destroyed when in the same scope due to persisting object references..