Hi,

Finaly, I could upgrade from DNN601 to DNN621 correctly, ouf ! I explained below why i couldn't see my skin the first time after upgrading. Hope you will understand my words (translation) !!!

First upgrade where i didn't see the darknight skin after :  
before starting DNN to execute upgrade, i replaced all the DNN files from 601 to 621. To do that, rather than deleted all files first, and secondly copy upgrading package, i replaced all and checked the box "always replace existing files". This is an important difference between both methods due to files permissions, i understood that later... :

Second upgrade where i could see darknight skin after :
i retried my upgrade to see if i always get my skin error : for this time, i deleted all files before copying updgrade package. Strangely, it was difficult to delete dnn files because i got many folders locked for unknown reason (Skins folder and others). After copying upgrade package (with correct permissions here) and executing DNN to upgrade, i could see my skin normaly.

So, during upgrade, i think it's very important to proceed in two steps : deleting files first, and second copying new files version, rather than replace all in only one step. Because, for only one step, permissions are not well reaffected.