Hi Dimitri, can you please post the 3DM model file with your object in it? That would help to give any tips on what might be going wrong. Just looking at your screenshot it doesn't seem like it should be a problem to fillet but if for example your 2 crossing shapes were just slightly out of alignment with each other where 2 edges were crossing each other that could have created a tiny edge when they were combined and filleting can be sensitive to stuff like that. It's very difficult to see in a screenshot if you have a case like that though.

- Michael