Symptoms
The end-user faces 404 error after a second login to OWA.
In IIS logs the following records can be found (formatted for better view):
2017-04-07 16:59:08 109.224.234.11 GET /owa/auth/errorfe.aspx owaError=ClientError;exMsg=Sys.InvalidOperationException: You are trying to get an instance of the UserConfiguration object before it is loaded from the server!
The error can be faced in Microsoft Edge and Chromium-based browsers (Chrome / Opera). The error is not reproduced on Mozilla Firefox.
Cause
There's a conflict between the Exchange CU and the latest releases of Chrome, which leads to the error. It is fixed in the latest Exchange updates.
Resolution
Install more recent Exchange updates, referring to the list of supported Exchange CU versions:
#8851 Supported Microsoft Exchange Server Versions