Does the tenant of a flat have any rights on the interests matured on the "caution deposit" or do they belong to the landlord? If yes, according to what percentage should the interests be calculated?
According to §551 BGB, the interests on the deposit belong to the tenant (except some cases, f.e. student dorms). Landlords have no rights on it (and they must save deposit separated from their own money)..
The percentage is just a usual bank percentage, so maybe 0.5% a year nowadays or even less.