Hmm, interessting. We also use Dell (T530, T630, T730. R430, R530, R630). Maybe this week there was an T730. Must have a look. But had no problem to install 4.1, 4.2. Have you testet the 4.1? In 4.2 Installer there was something changed. Had problem on new HPserver G9 (ML10, and some other little servers), but no on Dell. T730 is linux certified (no secureboot, no turstmodul...)? You must say that at order.
Maybe an incompatible hardware? Is something special build in the server?