I increased the swap size from 2 to 4 GB, and the rebake worked.
So, I guess it was about memory, though I am a bit surprised by how much it asks (8 GB RAM + 4 GB swap), especially since older rebakes didn’t have any issues with no swap and the same amount of RAM.
What should I do if I want to monitor RAM usage and check for any issues? What metric should matter to estimate the memory requirements? Page views per day (5000-8000 + bot spikes once or twice a month)? Database size (23 GB)? 
