After having put Agriculture in the research slot I accidentaly dragged another tech onto it. The tech disappeared from the available techs just like it had been put into the queue. When I removed agriculture both techs showed up again in the available techs. When I dragged all techs over the agriculture tech they all disappeared and I could not even be able to remove the tech from the research slot. When I left the research screen and reentered all techs were correctly displayed and working.