The loading message can occur in a front-end back-end OWA environment when a service pack or update is applied on the back-end before it is applied on the front-end, refer to https://support.microsoft.com/kb/910119/en-us for details. Any OWA updates must first be applied on the front-end server to ensure compatibility with the back-end server.

If no updates were applied and you are experiencing this issue, refer to https://support.microsoft.com/kb/280823/en-us for more troubleshooting tips.