iPxe, http, basic auth and login command in embed script and in menu script

[NiKiZe]

Good, so the important thing is to use the same dataset when doing debugging, printing ${username} and then expecting ${username:uristring} to have data, without checking username:uristring instead.

Which version of iPXE are you using, maybe it is an old build without uristring support.
Or it is something special with that, so copying it to a real string, could give different data:
set lusername ${username}
echo ${lusername:uristring}

but also try to create another string with data and try uristring on that.