It turned out that the power supply on their server was malfunctioning sometimes delivering too little power. Specially when taking the code paths my IP phone triggered.
The server software was built in php. It's not often you start looking for a bug in PHP but end up switching a capacitor in the PSU.
My point is that even if your write in php, php is running C libraries, that is running ASM that is running hardware and every part of this chain is important. There's no such thing as "works everywhere", it's just "have a very high chance of working everywhere".
(off-topic) thanks for the sds library. I'm a heavy user of it.
About SDS: glad it is useful for you!