To fix old Uplay "Log in" pop-up, follow this instructions.

(If you have a Windows PC)

Step 1: Install the game on your regular PC and start the game

Step 2: Go to C:\Program Files (x86)

Step 3: Zip the folder "Ubisoft"

Step 4: Upload zipped folder to anywhere you want (i personally use Telegram)

Step 5: Go to Desktop Mode on Steam Deck and download the zipped folder you uploaded

Step 6: Install Rayman Legends on Steam Deck

Step 7: After you started the game, wait for the Uplay pop-up and then close the game

Step 8: Go to Dolphin (File Manager), unzip the file you downloaded and copy the "Ubisoft" folder

Step 9: Go to "~./local/share/Steam/steamapps/compatdata/242550/pfx/drive_c/Program Files (x86)" and rename "Ubisoft" to whatever you want (i renamed mine to "Ubisoft Backup")

Step 10: Paste the copied "Ubisoft" folder

Step 11: Open Rayman Legends again and enjoy the game :)

(If you only own Steam Deck)

Step 1: Go to Desktop Mode on Steam Deck

Step 2: Download Ubisoft Connect

Step 3: Right-click on the .exe and select "Add to Steam"

Step 4: Open Steam and locate the .exe you just added on your Library

Step 5: Execute the .exe on the Library and just go "next-next-finish"

Step 6: Log with your Ubi Account and close the app

Step 7: Open Dolphin (File Manager)

Step 8: Go to "~./local/share/Steam/steamapps/compatdata/"

Step 9: Locate the last created folder and open it

Step 10: Inside this folder, go to "pfx/drive_c/Program Files (x86)"

Step 11: If you didnt find a folder named "Ubisoft", you're in the wrong folder. If you found it, then open "Ubisoft" folder and copy the directory path

Step 12: Install Rayman Legends on Steam and boot it up

Step 13: After Uplay screen appears, close the game

Step 14: Go to "~./local/share/Steam/steamapps/compatdata/242550/pfx/drive_c/Program Files (x86)" and rename "Ubisoft" to whatever you want (i renamed mine to "Ubisoft Backup")

Step 15: Now right-click inside "Program Files (x86)" and select "Create New -> Link to File or Directory". On "Name for new link" field, write "Ubisoft" without quotes, and on "File or directory to link to" just paste the previously copied directory path". After that, you should see a new folder name "Ubisoft".

Step 16: Open Rayman Legends and enjoy your game :)