QGIS Generating Lat/Long Degree Grid in Composer?

I made a map in WGS84, which is positioned and referenced correctly.

Now I loaded the map into the Composer and added a grid, with horizontal and vertical lines each 500 world units (m).

When I activate “Draw coordinates” and set them to Degree, it shows absoluteley weird coordinates, like first vertical line 120° E, second line 90° W, third line 160° E and so on, although the coordinates should be something like 120°1’0″, 120°1’1″ 120°1’2″ etc.

Also, the grid does not change if I set its CRS to WGS84, and it does not change, if I change the scale of the map, which I added to the composer.

Can someone can help me?