Hard sunroof crank

Sunroofs are generally a specialist job - I wouldn't attempt to get into mine. There is very little maintenance you can do yourself, unless you're very brave!:)

Lubrication 

Ok. I will give it a try with "white grease" Jadalber

The manual sunroof on my 240 displayed exactly the same symptoms as yours recently, including the clicking noise, and was impossible to shut without 'assisting' the roof from underneath on the last bit where it lifts into line with the 'roof'. I too was having doubts about taking it apart, but I found the problem easily. I don't know what the access is like on an 850, but for mine, I took the handle off, and the trim panel which is under the handle, this exposed the winding gearbox. Inspection of the gearbox, in situ, showed that the two cross head machine screws which hold the 'halves' of the gearbox together were very loose indeed (about 3 or 4mm loose!) I tightened these up again, and the roof worked perfectly, so obviously the slackness was responsible for both the general stiffness, and allowing the gears to drop out of mesh when under load. I then removed each screw in turn, and refitted it with a drop of the wife's nail varnish (no loctite in the house!) on the bottom of each thread to hopefully prevent a re-occurrance. Might not be your problem, but worth a look, 10 minute job if it is the fault. BTW I have bought some excellent'clear' grease which I find very good for sunroofs etcts not cheap, but can be had from engineers merchants, and is designed for lubricating the operating screw INSIDE water taps, so very, very clingy indeed, and colourless, so less likely to stain. I'm sorry that I can't remember the name, but engineers/plumbers merchants should know what it is.

I've got a complete electric one if you want it ;)

Loose screws