Why do I get an "ErrorID: 512 invalid clientID" even though the ID is correct?

Question

I've got a php TeamSpeak Bot for quite a while and for one user I now have this weird phenomenon, that the clientID is invalid.

So I store the clientIDs (TeamSpeakUUID) in a MySQL database.

As example:

I fetch the TeamSpeakUUID from the database and want to poke a client on my TeamSpeak with the TS3 PHP Framework (https://github.com/planetteamspeak/ts3phpframework). For every other client it works expect for one and I have absolutely no idea why.

snippet for the example:

while($row = mysqli_fetch_assoc($result)) {
  $client = $ts3_VirtualServer->clientGetByUid($row['TeamSpeakUUID'], true);
  $client->poke($msg);
}

I have two UUIDs here

Working: sP0GjP/pVyyfx07MvN+S1Cvt9mc=

Not working: a/1nYOo3Ej3afLa05ElVtxYDOMU=

I don't know why but if I want to poke the client "hardcoded" it works...

$client = $ts3_VirtualServer->clientGetByUid("a/1nYOo3Ej3afLa05ElVtxYDOMU=", true);

Do you guys have any idea(s)?