URLs on the website work fine, then after a few minutes they all return 404.
I've found some of the 404s in the log files, and they take this format:
2018-06-02 17:26:44 108.xx.xx.xx GET /store - 443 - 172.xx.xx.xx Mozilla/5.0+(Windows+NT+10.0;+Win64;+x64)+AppleWebKit/537.36+(KHTML,+like+Gecko)+Chrome/66.0.3359.181+Safari/537.36 404 0 2 280
The 2 at the end appears to be a system error code: ERROR_FILE_NOT_FOUND
I cannot for the life of me work out what's causing this to happen. The site works fine, then just stops and 404's everything.
Rebooting server temporarily fixes it, recycling app pool sometimes temporarily fixes it.
Server
Windows Server 2008 R2
Latest updates
Website target framework 4.5.2
IIS 6.1 Build 7601 SP1
IIS app-pool framework 4.0
Single application app-pool (not shared)