I tried that, also I tried to play with configurations and CComment content plugins, but still the issue happens when enabling the K2 plugin. I will also try to ask on K2 forum and see if someone replies. I think it must be something related to my host provider, unfortunately I have little control on the database and on error log, so it's not very helpful, I will also contact them and see if they can help. I don't want to say something wrong, but I think this also happened before when I tried to move my local install of joomla online by converting it to a quickstart package, but at that time I thought it was something wrong with the export...
Anyway it's very suspicious it happens as I enable that component and disappear when i disable it. You said it doesn't interfere with backend, do you think it could be K2 checking for plugin related to itself and somehow it crashes finding something wrong with it? Also I thought about something related with db, but still have no clue what it could be...
I'll let you know if they tell me something
Unfortunately it seems I am the only one in the world with that specific error appearing in backend, eheh...
Thanks by now