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ledieujulien:
I think you're talking about separate things here. The notices (not errors) you see when debug is turned on just mean you don't have the language strings right. That is, you need to translate the strings _AM_GROUP and _AM_UNASSIGNED in the proper language file. As soon as you include the defines, the notices will go away.
As to the mysterious disappearance of the who's online block, that's a different story altogether. A common cause of this is the assignment of the block to a specific page. When you do that, the Admin > Blocks page returns to the default, which is to show the blocks defined to show in all pages. Scan the different pages and you'll probably find the block somewhere, assigned to a single page.
Cheers.
