Do you have a graph of preprocessing queue?

Right after 14:15 "history index cache, % free" jumps from it's normal value to 100%, which means that there are virtually no items left in history cache (all were successfully written to DB, I guess?) Honestly, I haven't seen graphs like this one before, but there might be a buildup of values before history cache — in preprocessing.

Added new screenshots. No problems... My brain feels bad )

In my installation, it happens with hosts from only one main proxy, but proxy has no problem too.

Would be nice to get proxy graphs in better resolution around 14:00 on July 31 (when the problem started on server side). There is something going on, but it's hard to tell, because data is likely from trends at this scale.

Yes, surely. In attach.

This is a passive proxy, right?

Yes. Another one in active mode with no problems.

Are you still experiencing the issue or is it all over? Could you show server graphs when it works as usual (roughly at the same scale as Perfomance.png)?

It still exists. 

Now, problem has disappeared. After restart maybe or work of housekeeper... It happens during 3 weeks. But what I do wrong? 

After restart of what? How does it look on the proxy side, the moment when the issue disappeared?

I have no idea what it could be, sorry...

After server restart, but it connected with end of housekeeper work. Proxy perfomance after problem attached.

After server restart

Oh, I see it now. It's barely visible.

It happens during 3 weeks.

Does the issue happen regularly?

Yes, waiting for the next coming...

After updating to 4.0.12 the problem disappeared.