Thank you for the thread. The issue I see here is that everyone proposes to check the length of the DB that the FB is trying to write into. In my case, the FB is made by Siemens and read/write-protected. All the DBs are also automatically generated during compilation, so I have no way of modifying this part of the code.

I noticed the proposal to delete OB121 (Programing error interupt), but that one seems to me more like walk-around rather then solution of the root cause.

It looks like it's the same addresses over and over, can you give any hints about what is at some of the locations?  For example the first 10 entries in the diag log, tell us what things those are in your user program / HW?

I think you possibly have a configuration error which may potentially be further worsened by basis library blocks being out of sync with the correct versions or you have a corrupted/wrong block in the basis library of that AS. 

For confirming all the Basis Library: Follow steps from the manual to update the library using normal methods and see if any blocks need updated.

If the basis library is all consistent, otherwise need to track down the underlying configuration issue.

I have checked the DB of the only block running this FB, which is the CPU Performance block. Over there I am a bit lost as the adresses in Diagnostic Buffer does not really point towards any specific statics. I presume, as you sad, an issue is in configuration, as we use the same library on several other PLCs within the multiproject and only single one reports this issue.

I have also contacted Siemens directly and requested a support, as the FB128 is anyway inaccessible for troubleshooting.

If we will find solution, I will post it here for anyone in the future encountering the same issue.

Examples:
buffer event 3 Module address 14852 - that is most likely the diagnostic address of an object from HW config. 
buffer event 4 Module address 626 - this is most likely the I / Q address of an IO card or peripheral device from HW config. 

Either you have a corrupted HW Config (which if you are saying everything else works) seems unlikely

or you have lots of fieldbus issues constantly

or you have a corruption of the Basis library blocks and you should replace them with fresh copies from the basis library and compile download again.

Anyhow if you supply tech support with your project and the diag buffer they should be able to solve it for you like you said.

For anyone with same issue, we found the solution:

It was actually super-simple, through Component View -> Blocks, we have deleted all Organization Blocks and recompiled the PLC. The issue was in corrupted OB and the delete/recompile the issue was fully solved.