UP plugins may conflict with this plugin by changing owner id.

UP plugins? You mean ones that let the player upgrade their bases by typing /up 2 , etc? On my server, only VIP's have access to that and they are immune from The Purge anyway ... 

Make sure all authorized players and building owners were offline for your N time. Also notice that plugin starts counting offline time since it was first loaded.

As I said though, solo bases belonging to default players that have only ever had one player in them have been standing strong on my server for weeks.

This plugin has been running on my server for ages.  

"Purge Timer Frequency": 900.0,​

Tell me: in what units is the timer? s,m,h,d...