Update Jellyfin.Server.Implementations/Users/UserManager.cs

Co-authored-by: gnattu <gnattu@users.noreply.github.com>

Jellyfin.Server.Implementations/Users/UserManager.cs

@@ -113,7 +113,7 @@ namespace Jellyfin.Server.Implementations.Users
         // This is some regex that matches only on unicode "word" characters, as well as -, _ and @
         // In theory this will cut out most if not all 'control' characters which should help minimize any weirdness
         // Usernames can contain letters (a-z + whatever else unicode is cool with), numbers (0-9), at-signs (@), dashes (-), underscores (_), apostrophes ('), periods (.) and spaces ( )
-        [GeneratedRegex(@"^(?!\s)[\w \-'._@]+(?<!\s)$")]
+        [GeneratedRegex(@"^(?!\s)[\w\ \-'._@+]+(?<!\s)$")]
         private static partial Regex ValidUsernameRegex();
 
         /// <inheritdoc/>