Jantje:
The system exists out of the MEGA powered via the Jack and connected with serial pin 0 and 1 to the serial port of a router (a hacked linksys WRT54GL).
Not saying this has anything to do with the issue you had, Jantje - but when you say "connected to the serial port of a hacked linsys WRT54GL" - you do mean you're doing level shifting, right (such as detailed here: WRT54GL Dual Serial Port and SD Card Mods - JBProjects.net)?