You may have get free busy issue where particular user was not able to see free busy in outlook and OWA.
You may have notice event id 4002 on Exchange CAS server for availability service. In event you found out http 400 bad request for user.
This issue is caused by token size.
The user cannot authenticate because the Kerberos token that is generated during authentication attempts has a fixed maximum size. Transports such as remote procedure call (RPC) and HTTP rely on the MaxTokenSize value when they allocate buffers for authentication.
If user token size exceeds maximum size then user will start facing free busy issue.
Usually this happens due to large number of group membership.
To find out current token value for user you can use below script and check maxtokensize.
If you find out maxtokensize exceeds then there are two solution for this.
1. Remove group membership of groups which users doesn’t need (Recommended method)
2. Modify registry key on Exchange to increase token size (Not recommended)
Token Size check script :- https://gallery.technet.microsoft.com/scriptcenter/Check-for-MaxTokenSize-520e51e5