For characters such as 滑 or 骨 the square in the bone part is to the left. On other devices such as Anki and Iphone i however get that to the right. This is a font issue but my question is, is it an error in the font or is it ok to write it like this?

It's not a big deal either way. The simplified style is to have it on the left. The traditional style is to have it on the right. On Skritter, for consistency, since 骨 is used in both simplified and traditional without a clean separation, we standardized to having it on the left. It's easier to write as it's only one stroke instead of two.

We recently did just change 咼 to have the box on the right, since it's traditional only (呙 in simplified).

As I recall it's a style difference. See this thread for more details:

http://www.skritter.com/forum/topic?id=31107491