There is an incident that suddenly for 2 hours one of my moodle sites just can't be signed into. PHP error log during the time shows a few "Can not generate key-pair" and a bunch of "Disable login" and "file not found" error. On Windows event log there are 2 warnings about being FailedRequestTracing during the time. I checked IIS to confirm that its setting is correct. There is no other errors on Windows Events log during that time. The problem corrected itself after.
It seems to be a security key issue? I have a few sites running on the same IIS server. It seems only one of the Moodle instances is affected. Any suggestion in where else to investigate? I am trying to prevent it from happening again. Many thanks!
Windows 2012 R2, IIS 8, PHP 7.2x64