WYSIWYG not working in second step

My wordpress version is 3.9.1 and my ChimpExpress version is 1.6.2. After selecting the template and defining the campaign name and subject in compose mode, the next button is working fine but on the next screen, wordpress tries to open the editor but get stuck. Firebux says the following error “c is undefined”. If I ckick in next buttom firebux says currentContent is not defined

https://www.ads-software.com/plugins/chimpexpress/