Redoing all the piping I figured out that I had chemical plants with output of Petrolium Gas connected (built by robots).

Maybe a better error message is possible in this case?
Otherwise, problem solved.

When redoing the connection from my oil refinery pipe-system with my tank pipe-system both with light oil, I get the message "Cannot connect systems with different fluids". See first sceenshot. I cleand one system (refinery-system) and I still cannot connect the pipes. I think ther is a di...