It's a long time since I did this, brain cells and all that, but 'we' employed personnel via an umbrella company.
I happen to know that the umbrella company we were using was a shambles, registered in UK but with an offshore arm also, if the personnel worked in the EU then they were with the UK umbrella company paying a whopping GBP35, and bugger all tax/NI, per month for the privilege and if they were non-EU it was a small percentage of their earnings per month.
Utilising an umbrella company was ideal for us because it is the umbrella company employing the personnel and responsible for all legalities including employer's NI (Social Security), I was aware that other recruitment companies were setting up personnel as their own limited company or similar however the legalities of that, well my employer didn't even want to go there!
At the end of the day it is the individual's responsibility for paying their NI/Social Security and income tax, perhaps the authorities believe they have a case against Brookfield and/or whoever the legal employer may be for not deducting these at source, there is only one party responsible for paying employer's NI (Social Security) and that is the employer ... Although the German authorities would have nothing to gain if employer's NI was appropriately paid in to the UK National Insurance system!