Keystone: not Opening ./skin/keystone/Video.png: file does not exist
This applies to all different OS and MXWendler versions lower than 5.2
You open a keystone file or project with MXWendler and an error message appears that says not opening ./skin/keystone/Video.png: file does not exist
Ignore this message. This is not a technical problem. All keystone and other functionalities are still available.
You saved an .mxw file with Version 5.2 and higher and opened it with a previous version. Version 5.2 and higher change the internal storage format for keystone files. Versions 5.0 and lower are looking for helper textures, in this case, 'Video'. The 'Video' helper texture does not exist in these versions.
Among different versions, forward compatibility is guaranteed, backward compatibility is not. Keep this in mind when programming systems.