Did you try starting the dryer with the vent off? If it vents via a crawl space to the outside, just leave it unconnected into the crawl space for a load and see what it does. Maybe the sludge is from a blockage in the vent pipe. Could be the vapor accumulated when it couldn't get outside and mixed with the lint created a sludge. If it would vent into your house while disconnected you could see if the dryer heated up and had hot air/steam leaving the dryer.
These are just suggestions from a dryer owner. I am not a repair person, nor did I read the inital post about what sort of trouble you had. I called a repairman recently because my dryer was not performing very well. He said the vent had too many elbows in its' path from the dryer to outside.....that a dryer that couldn't get the hot air out couldn't get the clothes dry.
sorry i missed your earlier post, but no this is not normal. it ussually is a sign of a vent that is longer than needed and not drafting to the outside , in other words your dryer cannot push the lint and air out of the pipe to the open air, try using a shop vac to clean out what you can, then reverse the vacuuum and use it as a blower,, put vac hose into vent pipe, plug around it with a towel and turn it on. check the outside outlet, being an apartment, you may have to get maintenance to help.. if its stopped up, or the cover isnt opening thats the trouble area and needs fixed,,, you should be able to tell quickly if the blower is exiting the pipe or building up pressure,, pressure means a blockage and you need more help hope this helped
