CaptivePortalLogin has stopped

Hello, We configured an extarnal splash page on a Peplink AP AC mini, it works and the user does log in, but a after about 40 seconds in, the session breaks and it shows an error saying “CaptivePortalLogin has stopped”, this happens on android, on ios devices it only shows a Null alert. What could it be? thanks

Can you provide the screenshot of the error message? It just happens suddenly when the user is doing browsing?