I just found a solution. In the list of error codes that can be found here http://en.support.guildwars2.com/app/answers/detail/a_id/8994/~error-codes the solution is not listed under code 42 X X X but under code 21 X X X
Here is the solution :
add this :
/clientport 80
at the end of the shortcut’s target line, like this :
“C:\Games\Guild Wars 2\GW2.exe” /clientport 80
Couldn’t log on for 2 days. This solution worked for me. thanks