- Plesk 12.5 for Linux
Clicking on Check Security in WordPress displays that all the WordPress instances are not secured (marked with a red exclamation sign)
Opening any of a "not secured" installations shows that actually security is OK. After that WordPress instance is marked as secured (green checkmark)
After another security check, the instance is marked as not secured again.
This is Plesk internal issue with ID #PPPM-4247, that has been fixed starting from Plesk Onyx 17.0.
Too many WordPress files: the number of files in WordPress directory is more than the maximum allowed number of open file descriptors.
Consider to upgrade Plesk to Plesk Onyx.