Dimmer Raffstore "Text [%d %%]" { http=">[*:GET:http://hp27/deviceajax.do?cid=9&did=1010042&goto=%2$s&command=0]" }
In your item definition, between GET: and http://hp27 you have a [ which must be removed.
I notice that this additional [ was my mistake. Just make sure that you remove it.
Well spotted! That was very well hidden and I’ve carefully copied it too even while looking at binding docs …
I wrote a little application in Java to receive the command to make sure that the command was actually sent. Then I noticed the stack trace in my openhab.log log file indicating that something was wrong with the URL… The command is ok now, if I move the slider then I see the different values appearing in the java application. So now it should ‘just work’ for @usr 
The item Raffstore does not exist.
2019-06-17 08:57:45.459 [INFO ] [rest.core.internal.item.ItemResource] - Received HTTP POST request at 'items/Raffstore' for the unknown item 'Raffstore'.
2019-06-17 08:57:45.587 [ERROR] [ui.internal.items.ItemUIRegistryImpl] - Cannot retrieve item for widget org.eclipse.smarthome.model.sitemap.Slider
how can I get it?
Does your .items file still exist and does it contain the Raffstore item?
Dimmer Raffstore "Text [%d %%]" { http=">[*:GET:http://hp27/deviceajax.do?cid=9&did=1010042&goto=%2$s&command=0]" }
Oh great, anyway it works now. I am so happy!
Thank you @marcel_erkel and @rossko57 for your great support!
The community here is great.
Super toll