[CB 3.1.2] view lookup error

Occasionally in my logs I’m getting view resolution errors, such as

Could not find the file /views/custom/site1/user.cfm

Now, that custom view (set in conventions) doesn’t exist, but the external view does – so it should resolve to the external view.

It seems to be session-based somehow, in other words, a user will continually get it, and the only way to stop it is for them to logout and log back in again.

Anyone any ideas?