Artifacts lightmapping

Hello, I'm a beginner in lightmapping and I have some artifacts problems, when i do my lightmapping i got some gray or white "bubbles" (i've done some research but it seems that's artifact can come form numerous things so i decided to post my question).

Here are some screens

Here is the settings i used for this render

The two screens below are the first render i tried (so don't pay attention to the fact that it's tottaly burned)

How can i resolve this issue? I think it might be the resolution or the material maybe.

If you need more information let me know. Thanks for your help