1st of all, I hope that the translator did well enough...
In the case of mechanical / fabric production, it cannot be avoided that there are certain product tolerances. This is the case everywhere. But all this is no reason to make such big complains.
If something like this should accumulate unhappily / statistically for a user, then it is almost like a six in the lottery. But then you should contact the manufacturer over the usual support channels (in this case RME support e-mail) and ask if there are other possibilities in this particular case besides getting stuck on the repair costs yourself. Maybe you could have even avoided the repair and the involved costs, by finding other solutions together with RME.
How this will turn out now, of course, if you attack the manufacturer in the user forum ..
I am also a very satisfied RME user since years, nothing broke up to now (2 UFX+, 2 Octamic XTC, RayDAT, ADI-2 Pro FS BE, ADI-2 DAC, ARC USB and a few more that I upgraded/sold).
One should also mention how many people still use Fireface 400 and 800. And even those older interfaces can still be sold to a good price because of driver support and quality.
X10SRi-F, E5-1650v4, Win10 Pro 1909, Cubase, UFX+, XTC, ADI-2 Pro FS R BE, RayDAT, ARC USB