Check it out :)
Version 6.10.4

1. Enabled visits
2. Signed in with two users Admin and A Friend
3. Visited Admin from friend Logged out A Friend, checked visits and had same problem Image 1
4. Whilst still logged in as Admin checked User Friend and had same problem Image 1. checked Admin visits and all ok Image 2

Version 6.10.4
Update
Repeated above post #2

1. No change
2. No record of second visit from friend
3. Visited friend from Admin, go to check Friends visits and same error as original reported

We'll check that on monday.

@saunders @amoun Fixed in commit https://github.com/b2evolution/b2evolution/commit/23d0366e5537927bac03835c3f5eaa540090d5fb.

@yurabakhtin @fplanque Thanks, fix works great.