I just found out why this is:
- CPU Miner v4.0 does apparently not auto sense when to switch to the new PoW7.
You have to run it without the "-pow7 1" flag until it's time. Then restart it with the "-pow7 1" flag.

** The switch to PoW-7 will occur at block-height: 1,546,000 **
Block 1,546,000 is due to occur sometime during 6 april 2018.

You can check the current block-height at https://chainradar.com/xmr/blocks or other block explorers.