Cannot access Email module in the control panel

Hi,
Hopefully, someone can help me troubleshoot this. After logging into ISPConfig, when I try to access the Email module or the Email tab I get the pop-up box with the spinning circle for a while, the links on the left change to the Email links, but nothing else happens.

Everything else seems to be accessible and works fine. I have restarted ISPConfig and have also upgraded to the latest version. I have not been able to find a log that reports any error. I have not been able to find a thread/post where this has come up before... I could just be searching for the wrong thing and in the wrong places.

Thanks very much in advance,