Ok, i fiddled around this evening and found out, that the accesspoint works EVERY TIME, when using:
8 caps letters for SSID and 8 caps letters for the password. Example:
[[language]] M589 S"MEINDUET" P"MEINWIFI" I192.168.178.250;
As soon as the SSID contains lowcase-letters or numbers, it doesn't work anymore. Same for the password. And using less or more digits than 8 doesn't work too.