While saving SaveWorldState() has thrown "Exception during world save", what can the possible cause be?
POL was still running and we tried to delete last items created but we were still unable to save

when shutting down in the console was written this:
SIGTERM: exit
Writing data files...Server Shutdown: writing data
Leftover objects in objecthas: 321424
Leaking a copy of the objecthash in order to avoid a crash
Execution aborted due to: St9bad_alloc

I had that when we were hosted on a server that shared its cpu with other web applications and those other applications were very sometimes resource hungry when they were running.

I would be considering external factors to POL itself - look at RAM usage, hard disk space, slightly faulty ram etc.

We had this problem on average every 2 weeks while we were on a particlar server hardware. It vanished the moment we got a new server dedicated to pol.

we are on a dedicated server, i don't think it's for that

Your error makes me think of corrupted memory though: "bad_alloc"

I don't think it can be a memory corruption, because that error should make unusable the whole system, not only POL saves...

Memory problems can cause total system crashes but they also cause strange errors too. It depends where the bad memory location is. If it's not in the area of RAM occupied by the OS kernel but is in an area used for say data storage then it could be a memory issue.

Heck, I've even seen video card failures cause corrupted data to be written to the hard drive.

Like Scotty says: "The more they overtake the plumbing, the easier it is to stop up the drain."

Every other application on the system, except POL (more specific: POL saves), runs without problem... and the server was used in last month as a stable web server

I understand.

I had a bad memory module once and my computer ran just fine until one day I was running an application that needed a ton of memory. I began having strange crashes and after about an hour of debugging I decided to remove the second (newer) memory module and see if that was the problem. I had to deal with a slower machine due to virtual memory swaps but it stopped crashing. The bad memory location didn't manifest itself until I began using a ton of memory space. Prior to that one application I was probably not even touching the upper limits of my memory or maybe I was just lucky for a while.

Depending on what OS you are running a webserver may not use much if any RAM, especially if it's a Linux based server. However, once you put POL on the machine and especially if you've a fully spawned world and multiple maps with a fair amount of players that will use a lot of RAM.

Now I am not saying it is a memory problem mind you. All I'm saying is don't dismiss the possibility that it could be. If you can swap in some different memory modules from working systems or maybe do what I did and remove one and see how things work. If all seems to be fine then put the module you removed in and switch out the other one.

The truth is it could be anything including a POL bug true enough. Maybe there is an application conflicting with POL even.

I wish you well at any rate.