Hi, I have tested version 2.5.4 on the server where this problem occurs and
this function get e.g. 0755 in the $folder_permissions variable
@JFolder::create($folderThumbnail, $folder_permissions );
so it gets the following
@JFolder::create($folderThumbnail, 0755 );
but it doesn't add the 0755 ??? I have tested to check it for the integer
@JFolder::create($folderThumbnail, (int)$folder_permissions );
but then I got problems on servers where there were no problems, so the integer checking is no solution
maybe there is something in JFolder::create method what changed it, maybe there is something on the server ???

, maybe the type of the value (0777) is somehow wrong
