I don´t find the TX ID in the Blockchain-Explorer.
31b3e23780cf5e459ce8b2baeaf3582de97e1b412a8357b191d0a58f0beb341b
How can I solve the problem?
Blockchain.info is just one of many block explorers, and it ignores transactions with low fee. There is no "one memory pool".
Your transaction does show up on
blockexplorer.com. At 20 satoshi/byte, I would have expected blockchain.info to show it though.
At this low fee, it's very unlikely to confirm. It's best to abandon the transaction and try again with a higher fee.