The UK diplomat is said to have been referring to complex Brexit negotiations rather than the country's decision to leave the bloc, which he backed during the 2016 referendum, The Telegraph newspaper reported.
The meeting is understood to have either taken place during Johnson's November 2016 visit to Berlin or during another visit in April 2017.
Brexit negotiations between London and Brussels started in June and are due to be completed by the end of March 2019. The border between Northern Ireland and Republic of Ireland was among the key issues discussed during the first phase of negotiations, along with the rights of EU citizens residing in the United Kingdom. In mid-December, EU leaders agreed to move on to the second phase of Brexit talks, which is to begin this March.