certain users being randomly(?) logged off

I have a handful of users who report being regularly logged off Sugar.

They do not run API requests with their credentials, nor do they log into multiple browsers/computers, so it's not a case of multiple sessions with one set of credentials.

They are not doing anything particularly different from others. Everyone uses LDAP authentication to sign into our Sugar instance.

The error they get suggests an authentication issue, but they are simply navigating, nothing more.

There is no admin work going on at the time, not via studio or otherwise, so no QRRs or clearing/rebuilding of cache anything like that happening at the time.

It's always the same handful of people (out of 150+ users), and I can't find a common thread with what they are doing at the time, or how they are doing it. When this happens they simply reload and get the login screen, after which they can simply log in and move on. 

We are still on Professional v11.0.1 (yes, I know... working on it!) on Site.
Some of the users are on site at the office, others on VPN working remotely.

Any thoughts on what to look for?