Hi,
i did not enabled plugins and I did'nt noticed any other install.
The only thing I have done is:
created 10 artices with a publish date set to future dates.
After I noticed the error with the plugin I have set the published date for today.
So i think this can not be the error.
I did not made server configuration changes - same as it was the last month's.
To be honest, i am totally clueless with this problem.