No- she asked for euro 150 security deposit nothing else
That will be, in many hotels, on top of pre-authing the bill. The security deposit is breakages, room service and mini-bar.
I don't deny there has been some mis-communication here, and perhaps they didn't explain properly, or you didn't hear in the noise in reception, or whatever. But it is absolutely common for hotels to pre-auth the room stay as well as taking a deposit for incidentals, and in fact in a lot of German hotels (Motel One is not alone in this, but is unusual in doing in the UK as well) you pay for the whole stay up front.
Am I to understand that if I want a treat in a hotel in have to have the money in cash as well as in my account?
If you are going to check in with a debit card, you need to have the money for the bill in your account, and pay with that card when you leave.
Or get a credit card.
What happened is that you checked in using a card against an account which contained your rent (or whatever), and the hotel pre-auth'd successfully because there was enough money in the account to cover the bill. It's neither their, nor the bank's, fault that you had that money earmarked for other things.
This is why I never understand the "oh, I don't have a credit card because it will get me into trouble" thing. I would never check into a hotel with my debit card for precisely this reason, and that goes double for the debit card on the account I pay bills from.