It turns out that a FTP download call using winscp as last thing to do in the batch caused the problem. After inserting the echo command it works fine. Guess the problems source could be the winscp.exe which do not correctly report the end of the current task to the OS.
del "C:\_ftpcrawler\Account Export.csv" /S /Q
"C:\Program Files (x86)\WinSCP\WinSCP.exe" /console /script="C:\_isource\scripte\data.txt"
echo Download ausgeführt am %date%%time% >> C:\_isource\scripte\data.log