I use UCCx and Agent desktop version 5.0, build 188.8.131.52 and it works for me now.
The open in new tab option is greyed out, but if I use the open in new window option it opens within my agent desktop just fine.
Does anyone know when tabbed browsing will be available in UCCX. I wanted to open up 10 websites for the agents at startup and have them all ready for them. I tried to create the http action for startup but it only reads one of the steps no matter how many I have enabled.