I had exactly the same problem Dakatine describes in his question 29 May 2014, with QGIS version 2.10.1. I used a raster map provided WMS from Finnish Land Use Service (an authority providing WMS).
Normally I get the cascade of maps from server and the program chooses the right scale map automatically when I zoom in or out on my display. This automatic function does not work properly with print composer, as Dekatine described (the program just gets wrong map when you try to adjust the scale on map item tab - it tends to get larger scale map than you have written to the scale field, eg. I got 1:100000 map, when adjusted scale 1:45000 - I wanted 1:50000 map of course, which is closer to 1:45000 and more detailed).
I worked around this problem by connecting to wms again, choosing and getting the map of certain scale only (1:50000) and made it active on main display of QGIS.
After that, I opened the print composer and I got this map as background. This is not of course the way it should work, but when desperate, you can at least proceed with your work.