hello
does anyone know until what year/date the DSB MX and MY locomotives were used in passenger and/or freight traffic to/from Flensborg (Germany)?

When the first 4 My were placed in service in 1954, they did not go south of the border.
The schedule from 1955 called for My on "Nord-express" (both south- and northbound), but again: the change of traction happened in Padborg.
Not sure from when the Danish diesels went to Flenburg for change of motive power.
At least from 1961 the train "Nordpilen" had My between Padborg and Flensburg where a German diesel took over.

It is know that in the period from the summer 1963 to the summer of 1965 a DSB My powered train D152 between Flensburg and Hamburg (and D 147 from Hamburg to Flenburg). The lay-over in Hamburg were only about 1,5 hours so no need to consider service facilities for the DSB diesels.

Since both Padborg and Flenburg are regarded as border stations, motive power from DB and DSB could be seen on both stations.
I do believe once we get into the 1970s you can expect both My/x/z in Flenburg.