I've just been checking both TalkTalk Webmail and client access using both Android and Windows devices and seems ok here.
There is a redirect issue on logout and the Webmail browser is indicating a security issue but don't worry about that. It'll be fixed and in the meantime just close the browser page tab and open a new tab to select your next browser page.
Are you using Webmail on the Windows PC? Have you temporarily disabled any ad-blockers / add-ons. Has your Windows PC had a software update recently?