So I have funded my Bisq Internal Wallet by another p2p Platform. 30 Minutes later a transactoin without my doing happened, which drained all the Funds…
Where did you download the Bisq wallet? Did you verify the installer file?
Check that this address was really on your Bisq wallet, and consider your computer at risk.
to expand: bisq wallet is a “normal” hot wallet, where the seed and private keys are stored on a pc which is directly connected to the internet.
It is either possible you downloaded a hacked version of bisq, from an unofficial source, which was showing you a wallet with a known seed (by the attacker who made you download that app), or that your whole PC is compromised to the point someone has remote access to it.
In any case, the same thing could happen with electrum or sparrow or any other wallet software that had private keys stored on disk