1. I had ArcMap open (I wasn't connected to the Postgres gDB this time; I don't remember if I was connected the last time).
2. I tried to restart the Postgres service after making a change to its pg_hba.conf, but got an error about the lock file postmaster.pid already existing.
3. I closed ArcMap.
4. I tried restarting Postgres again; I was successful.
5. I reopened ArcMap and tried to connect to the Postgres gDB, but got an error (attached) about how ArcMap couldn't load the st_geometry.so file (I'd had no trouble connecting to the gDB before getting this error).
6. On the CentOS server, I deleted then recopied the exact same st_geometry.so file from the Windows server onto the CentOS server that was in place before the error.
7. I tried reconnecting to the gDB in ArcMap; I was successful.
Any ideas why I get this st_geometry.so file error? I'd restarted the Postgres successfully many times after modifying its pg_hba.conf file; thankfully this error has only happened twice so far.