Helllo. Usually that means you have too many boss quests waiting. Only so many boss quests can appear at the same time. It can also vary based on number of mods you have installed. There is a mod by Mafe that allows a much larger number of quests, you can optional try that also.

Thank you for your answer. I will try and If I don`t reply in 12h it means it worked.

I found a reason of this. Spider Nests 2. Quoting one of comments that you`ve got there „this mods breaks the game if you start playing a new dungeon, after a few weeks the game gives nothing but spider quests with those god awful maze dungeons, With level 0 heroes it becomes impossible to progress, ill have to uninstall the mod, it's unfortunate because its pretty good”. After deleting Spider Nests 2 everything is back to normal.

that is the same issue, the Spider Nest boss quests were also being ignored, so no new boss slots could open.