Brake force gain failure

I've had a problem with my brake since yesterday.
When I was driving on the highway at 180, it suddenly started to bee and the symbol of the braking and hydraulic system (! ==References== I wanted to find a parking lot to stop, but after a few seconds it stopped again. The brake was also normal at first.
A few kilometers further I had to brake, but there was no more brake reinforcement available!!! The pedal was immediately rock hard and the braking effect was so almost zero!!!

Luckily, I still had enough distance to the front man and was not fast, so nothing happened.
I stopped right away and looked for the brake and hydraulic fluid, but both are still fine.
I've found out that the brake pedal gets tough right after the engine is switched off, so I guess the brake pressure memory, called bomb, is broken, right?
But can this also be the reason for the failure of the braking force booster during the ride?
In addition, the warning symbol with beeping also appears if you step the brake in a standing position several times in a row.

What can that be?
I do not just want to switch the bomb and hope that it has done away with it. If it happens again, I could be less lucky.

Who knows?

Burkhard