Thx all for the literature references. There are some very important issues in Constructive beekeeping written by Ed H. Clark. Yes, the bee system is really advanced as far as humidity management is concerned. Condensation is by far the most important mechanism to get rid of moisture. Winter mortality and absconding are clearly explained at the hand of humidity in the hive. 

But I question if condensation occurs sufficiently and fast enough during winter feeding, in particular when the sugar syrup is administered in large quantities or at once. Many beekeepers provide the winter food in small portions. Seems better to me, but do they also do this in dependance of weather predictions (temperature at night and during the day)? Is there something to win here?

IMO too much humidity in the hive is a condition to avoid. The scientific literature is contradictionary about the effect of humidity on varroa. My reasoning: if high humidity is good for reproduction of the bees (Clarke), then it is also good for varroa. Coincides varroa building up with winter feeding? The existance of viruses was unknown at that time (1918), as was varroa ....
My statement for the moment: it is better to control humidity than combat varroa, which is an end-of-pipe solution. There is a couple of options.

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