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..