Hi Simon95,

I had a similar issue recently. Although I got "System coupling run coupled successfully", the progress was stuck and busy like the photo you posted. I also waited for at least 6 hours overnight but it was still busy.

After quitting the Workbench using Windows Task Manager, I had to rework the dynamic meshing since I forgot to save the file before system coupling. But this time I changed the step size from 0.005s to 0.01s. The "stuck issue" was somehow fixed after these actions. 

Having to use System Coupling for the first time, I do not know the cause or what helped solve this issue, but I hope that what I have experienced can help you somehow.