Thank you all for helping me with this problem.
This worked for me;
https://community.spiceworks.com/topic/514617-exchange-2013-unable-to-login-to-owa-ecp
This is the solution:
Using the commands :
Set-OWAVirtualDirectory -Identity "adminsrv\OWA (Default Web Site)" -FormsAuthentication $False -BasicAuthentication $True
Set-OWAVirtualDirectory -Identity "adminsrv\OWA (Default Web Site)" -FormsAuthentication $true -BasicAuthentication $True
Followed by IISreset and also to recycle the app pools for OWA and ECP