It seems I am going to close this thread. After several attempts, I manage to figure out that when tabSRMM is configured to generate notifications (and I remove MessageNotify, or NewEventNotify plugins), chat popups start to appear. The only sad thing is that the whole system (architecture) is so confusing that it requires trial and error approach to get things set up correctly. I have not found any info neither on forums nor in miranda about proper configuration of group chat.

It's not confusing nor anything else. You just need to understand that Miranda distinguishes between group chat and normal chat sessions. These are two complete different things and this is not related to tabSRMM or any other messaging window. In fact, by default, you must install TWO messaging plugins to get support for normal and groupchats (srmm.dll and chat.dll) - tabsrmm has this merged into one plugin and same goes for scriver. But these 2 parts still must be configured separately and there is nothing I or anyone else can do about it, because this is just how it works.

Edit:Yes, we are aware that the separation between 1:1 IM and group chats is ugly and must go away. A more unified messaging api will come at some time and should hopefully eliminate the current difficulties people are having in understanding that they basically have to setup 2 completely separated systems.