I'm doing my tests. I see now that in the upgraded CB Activity (Tab, Module or direct link page to activity) the joined site activity is reported this way "username has joined [sitename_linked]". So instead of the site name is shown [sitename_linked]

CB Activity has changed quite significantly. I've rewritten it from the ground up. Please see the below regarding some of the changes. It is recommended to purge existing activity. I am working on a new CB Auto Actions release that'll come with Default Actions to track activity.

www.joomlapolis.com/forum/6-news-and-publicity/229590-cb-activity-30-rc1-released#263605

Second problem. With CB 1.9.1 we builded a canvas profile image, now we would like to migrate those images to the new canvas layout field. Is that possible?

I've no idea. We have nothing in place to account for this. The canvas usage is just an image field so I suppose if you stored their canvas in your custom implementation as a field then you could do an update query to update the canvas column with your other image fields value.

I've also activated the maximum error and I have an error with an old plugin that should be rewritten for CB2.0: Notice: Undefined property: CBframework::$_ui. How could I solve that issue in the plugin code?

You need to provide the full error as I've no idea where that's coming from.

Finally, I'm testing the JFConnecto template and I have a blank page when I go in backend to fields or tabs: Fatal error: Cannot redeclare class cbTemplateHandler in /home/myaccount/upgrade/components/com_comprofiler/plugin/templates/jf_connecto_cb_20/jf_connecto_cb_20.php on line 922

I think that was fixed in their latest release. Regardless we didn't develop that template and can't help you with it. See the below topic regarding that template.

www.joomlapolis.com/forum/6-news-and-publicity/230010-template-providers-and-community-builder?limitstart=0