I'm getting this error mesage when I try to install a new plasmoid or anything else from plasma throught kde desktop. I'm using plasma 5.3 on kubuntu 15.04 and have already tryied "apt-get install kio". Is there a way to solve this bug? Thanks.

Something manipulates your http headers - could be a broken (invasive) proxy or a zealot firewall - or kio-http or one of its dependencies is broken.
Can you reach www.kde-look.org from any browser?

Despite being a stupid solution: try to re-login (if you didn't already) - could as well be spurious due to an update and incompatible plugin versions (error 203 is rather opaque)

http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html