I’m not sure there’s any way to deactivate LeadPages really, isn’t it a complete theme? If possible it might be worth doing to see if it is the cause, and of course if all works well using one of the default WordPress themes it would indicate that it isn’t a Simple Press issue, and actually a LeadPages thing.
I could be off the mark, but if that’s the case you’d really need to ask LeadPages support, of course it might be worth doing that in any case!
Although possibly someone else has more experience with LeadPages, I’ve never used it and a quick search on the forum here looks like we’ve never dealt with it before..
Sorry I can’t be of more help!