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The server has 4 cores and 8 GB RAM, about 10 Joomla sites, different versions, 3-4 of them with lscache plugins. PowerDNS, Postfix, FTP, CSF and modsecurity are enabled.
After sometime OLS is not accessible at 7080 port. I can access CP on 8090 and restart it, so it is functional again.
What can be the reason? Where should I look?
The Chrome error is
This site can't be reached
... took too long to respond.
You have to add port 7080 to csf (ipv4 port settings) then restart csf for some reason it is not incuded as default.
I already opened 7080 port in CSF and it is accessible (for some time).
It was the first thing I did during setup. It is needed (for me) for (1) learning (2) fine-tuning (3) detailed inspection of connections during testing. I may close it afterwards. CP does not replicate OLS panel (it shouldn’t).
The problem is: it becomes inaccessible after some time - a rather random period.
I read about “number of open files” problem in multiple places, did some touches but I don’t know what to look to debug.
Actually I don’t remember what I did 
Just re-checked the file descriptor limits as indicated, they are 65535 and it is working.