I'll add in one thing that happened to us back when first bringing Portal up.
After installing, we allocated added disk space to our Data Store VM (WinSrvr 2012 R2).
But, I forgot to use Disk Manager to expand the partition into the new free space.
Hence, the disk filled up when I wasn't looking.
I worked with Esri to try to recover but in the end it was much simpler to delete the VM and reinstall.
Having enough Disk Space is a pretty basic thing but sometimes it's the obvious that really bites us. We now monitor disk space. 😉