Hey there @Calvans_Kliens
First of all, it ain't a problem at your end. No need to Repair your installation or reinstall the game or anything, it's a server issue.
Chances are the game accidentally spawned you as a Cavalry in a tank. This is a notorious bug that has been around since the release. Sometimes you spawn as a pilot on a horse. Sometimes you spawn as a Cavalry without a horse. You're lucky that you at least got a tank since I've had situations where I spawn as a Fighter pilot only to find myself not having a plane to fly with ! Flapping my hands didn't help as I met my early demise hitting the ground with a lovely smack.
Anyways, my best advice to try and avoid this issue is that do not spawn your vehicle immediately after a round starts. Usually this issue tends to happen when a lot of vehicles are spawned at once and the server gets confused as to who should be given what class. Just reserve the vehicle of yours at the beginning, give it a few seconds, then spawn your vehicle. Lag also seems to be rather directly related to this issue. More lag, more common this issue is.
Sorry for this issue though, I do hope this bug would get fixed eventually.