Ihad no problems with the mosquitos inside either the hotel lobby nor in my room - even with the window open in "tilt-in" mode for the entire duration of my stay. The sheer gaussy curtain made a very effective mosquito net!
But on my walk-about on Sunday, I headed first down towards the rotary circle south several blocks, then towards the river - and somewhere along the line got bit, and it started to swell and itch almost immediately. Upon my return to the hotel and my room, I examined the bite;it was on the left side of my neck, where a vampire would bite in a horror movie. I had a welt that was over 5 cm long, and almost 1 cm high!
While on my exploratory walk, I happened to see a poster on a kiosk, advertising the town's annual BierFest - the ad prominently featuread an anthro female mosquito in a dress, sitting at a table and holding a stein of beir. She could have been the official town mascot, for all I know, not just a symbol for the BierFest!
I never got to see the river itself - apparently there is an island which has been turned into a pedestrian park or garden, suitable for strolling about - but if the River Elbe has a slack body with a few stagnant, rush-filled lagoons, that would be enough. I got the impression that the streets are not really graded for good drainage, so there may be plenty of standing water spots all over town for mosquitos to breed in. I spent 2 years in the Panama Canal Zone, aquired an immunity of sorts to the saliva of the local species after about a year there; it may be that the natives of Magdeburg get bit, but aren't bothered with swelling and itching.