Fritz
It appears the system doesn't like some ascii characters in the tail number string. (e.g. "." and "+")
When adding German serials use the "space" instead of "+"
http://www.airport-data.com/aircraft/98%2033.html
http://www.airport-data.com/aircraft/50%2098.html
When adding Spanish serials, do not use the "."
http://www.airport-data.com/aircraft/C14-06.html
It seems to work this way.
Also when searching for certain serials, you need at least 3 charachters;
searching for 50+98 you can enter 50 "space" 98
searching for all luftwaffe serials in the 50+ range you enter 50 and "space"
John