Hi,
How many users are affected by this?
Try to let the user logon OWA with a different web browser, or on a different computer, does this issue persist? This step helps determine whether it's a browser or client related.
I also found some similar issues in our forums. Such as this one below:
Try some suggestions mentioned in the thread, like "check the authentication settings for OWA and ECP in IIS", or "reset the ecp virtual directory". Check if they are helpful.
By the way, in the current forum we mainly discuss questions and feedbacks about Outlook Client, since this issue is related to OWA, for further assistance I suggest you post the question in Exchange forum:
https://social.technet.microsoft.com/Forums/office/en-US/home?category=exchangeserver
Regards,
Melon ChenTechNet Community Support