The two affected nodes have now been back online and responsive for a bit now.
We got an update about what occurred with these systems.
A data center technician was working on another unrelated system in the same server chassis that EU-NL-P17-A2-S and EU-NL-P18-A2-S were located on.
At some point during the work being performed on the unrelated system, there was a hardware issue with the server chassis that took down the two nodes.
When a reboot was attempted on the two nodes, the drives in them weren't detected by the BIOS and they couldn't boot into the OSes.
A chassis replacement was performed for the systems, after which the nodes seem to be back online and responsive.