I understood large sums of Monero can only be mined in several transactions? Could EUR 10.000.000 be mined in ONE transaction?
Firstly, the market price of Monero in EUR has little bearing on any of this.
Secondly, mined or transferred? The question is not clear. Regardless...
There is no cap on the value amount you can transfer (so long as you obviously own that amount) and the amount of Monero that can be mined is dictated by the emission schedule.
However, when considering large transfer value amounts, you presumably own a lot of separate outputs (thus inputs funding the transfer), and whilst there is no cap on the value amounts, the more outputs you spend (so tx inputs) the larger the tx storage size becomes. Therefore the constraint becomes the tx storage size, which can currently be no larger than 1000000 bytes (src). Furthermore, there is a maximum number of transactions that can be mined in a block, 268435456 (src).
Putting this all together, if you owned one output with one Monero and the market price for 1 XMR was 1m EUR, nothing whatsoever would prevent that transfer or it being mined in a block. If you owned 1m individual outputs you wanted to spend, you would have to send them in many transactions because of the tx size constraint; a miner can still mine 1m transactions in a block though.