Hi Luuk,

I had a similar issue with my BS4, the glass would not operate at all and now functions correctly.

I measured the original b&o broken belt and it is 100mm-101mm in total length and 25mm in diameter. I brought a kit of o-ring seals intended for plumbing at my local hardware store and found one about the same, it has since worked perfectly. I remember reading the belt needs to be tensioned so your belt may not be adequate enough at a 30mm. Try going for a 25mm diameter rubber belt with an circular inner core diameter of roughly 2mm (as measured on the original)

Only other things I can think of is the gearing system. The two sides need to be aligned, this can be done by changing the position in the gearing for each side (ie puling one side mechanism from the gears teeth and pulling it up or down, they need to both be level otherwise your glass will not be square.) Ensure the mechanism is at full height before plugging back in and testing.

It was a lot of work but it is fixed. I had bought the 25mm/2mm belt but in my case it was not working. The belt was to tight sow the glass panel wouldn't work at all. After that I had ordered a case with belts from Aliexpress. First I tried a 28mm/2.4mm belt, that belt was also to tight. The glass panel wouldn't move at al. At the third try I have used a 30mm/2.4mm belt. In that case the glaspanel of the system is fully working. Now the glaspanel works and also the lights on the disk player works good.

For someone that's interested in the link of the belts from Aliexpress here it is:

https://a.aliexpress.com/_EIERRx1

Here the company of the site that claims that they have the right belts. They have send me 3 types but all with the wrong measurements. After that three tries I had bought the belt case on Aliexpress.  https://www.electro-andijk.nl/categorien/cd-speler/cd-speler-snaren/beosound-4-door-belt-,-beosound-4-deur-snaar-ronde-profiel-detail