Thanksn for all advice. The problem has been solved. It seemed that it was caused by the 3-pins fans I used instead of the standard fans. The upgraded firmware has added protection against fan malfunction but didn't recognize my fans and thus shutdown the miner. 

Yesterday I upgraded my 9 Antminer S7's batch 1 with Bitmaintech's newest firmware release. After a restart 6 of them worked fine. But the other 3 first seemed to work at normal output (appr. 4850 Gh/s), but within a minute speed reduced to almost 0 Gh/s and remained there, while the red light on the back kept on flashing. There has been no way to get them up and running normally again (reboot/ reset nothing helps) and I don't know how to reinstall the old firmware again, because I don't have it and it's not in the support section on the Bitmaintech website. Does anyone have similar experiences and/ or solutions or can anyone help me to get the old S7 firmware?

Can the download for the new A2 mini also be used for the new A2 110 Mhs?
And many thanks for the good work!