Hello Anibal,
If the url is opened in a new window, it means that IFRAME element is not found. However, since the first time you open a page it works correctly, this means that the IFRAME is there and you have correctly referenced it. Another possible reason for the problem is that you are opening the second page in a different context - if so, please, make sure that you use the window object of the main page which includes the splitter.
Basically, this is not related to RadSplitter and RadControls and I suggest to remove the splitter and put a standard IFRAME element to replicate the issue. Once you resolve the problem with the IFRAME simply use the same logic for the splitter as well.
In case you need further assistance with this, please provide fully working sample code which can be directly build and debugged and I will examine it and I will do my best to help.
Kind regards,
Svetlina
the Telerik team
Check out
Telerik Trainer, the state of the art learning tool for Telerik products.