One small update regarding the situation with SM550 and USB-C frontpanel with type E mainboard connector:
I described the problem to as best as I could, and sent it to "orders@sliger.com", since that was the closest thing I could find to a support communications channel.
They were very friendly and replied "sure, we can help, we just need your order number". However, since I am located in Europe, I didn't order anything directly through Sliger, and don't have any order number. So after that I essentially didn't bother anymore. Not sure how that would have turned out if I would have kept at it a bit longer.
However, since it is only 20€, I eventually just bought a replacement front panel through density.sk
So, now I have two frontpanels, and I have to say the issue is the same for both of them. Front panel USB-C is only working in one of the four different cable orientations (turn cable at device side, turn cable at PC side). In three of the four cable orientations front panel USB-C is reliably not working. So essentially if you plug in a device you have a 25% chance of it working and a 75% chance of it not working, since USB-C cables due to their symmetry really do not give any indication on how they need to be turned.
It's also not about interference from other cables, since it is happening also with open case and detached front panels.
So now I have tested two different Sliger front panels and both have this issue. Meanwhile with the front panel of In Win 301C it worked perfectly, all other components being exactly the same (i.e. mainboard as well as cables and USB devices I used for testing). I have to assume that the problem is inherent in the Sliger USB-C type E front panel. Which is of course a bit ironic, since attaching a device to it is really the only possible use-case for a front panel USB slot. And that only use case is plainly not working.
tldr; Do not buy the front panel with internal type E connector for USB-C. It is not working.
