Mycelium: Restore backup from Android to iOS
After lots of suffering I managed to do it. Very unfortunately mycelium does not even support importing their old keys.

Fortunately they provide a backuputil which converts the private key in their format into WIF (Wallet Import Format): https://github.com/mycelium-com/wallet/tree/master/backuputil

I takes some effort to built this tool but afterwards you obtain the private key in WIF format. This in turn can be entered in the new Mycelium on iOS.

Again, very unfortunately, the key is NOT imported but rather the funds transfered to the Mycelium account. This results again in unnecessary transfer fees.