The club have failed to win any of their first five matches in Denmark's Superliga and sit second from bottom of the table.
Chief executive Anders Horsholt told his club's official website, www.fck.dk: "The results we have achieved on the pitch in recent months have not been acceptable for FC Copenhagen and therefore we have decided to discontinue doing business with Ariel Jacobs.
"We do not question his general abilities, personality and dedication. Our decision is based solely on sporting grounds."
Solbakken, now 45, was previously manager from 2006-11, helping the team win the league five times. They also reached the knockout stages of the Champions League under the Norwegian three seasons ago.
He has since had unsuccessful spells in the Bundesliga with Cologne and in England's Championship with Wolves.