How can I regain the Instant Hotspot feature in Yosemite after noticeable errors? While normal hotspot interaction functions perfectly between an iPhone 6 (iOS 8.4) and my MBPr '14 with Yosemite 10.10.4, the Instant Hotspot feature does not. This feature used to work, but after some time this feature stopped working. 
I found the source of the issue, but can't determine how to resolve it. 

My Yosemite logs reveal some errors.

com.apple.preferences.icloud.remoteservice[749]: [-[CastlePlugin appleIDNeedsFixing]]
accountsd[270]: Deleting com.apple.gs.appleid.auth service token
AppleIDAuthAgent[286] SERVER AppleIDAuth: Forgetting account "&lt;my-apple-id&gt;@?????.???"

SDHotspotBrowser::appendHashToData:withType: No apple ID hash
SDHotspotBrowser::restartAdvertisingTetheringRequest Did not start advertising request

Steps I've tried that have not worked:


Signing out and back in to AppleID on both devices
Clean restore of iOS to iPhone
Ensuring wifi and bluetooth enabled on both devices, neither with an active wifi connection
All the above steps with full LTE coverage and 12 inch proximity between devices
While other answers here may help troubleshooting other causes of instant hotpsot failure, mine was caused by having two iCloud accounts used for keeping some syncing features separate. 

Steps I took to get it working:


Turn on bluetooth and wifi on both devices
Connect both devices to same home wifi connection
Sign out of all icloud accounts on both devices
Restart both devices
Sign into both icloud accounts on iphone
Sign into both icloud accounts on OS X
Restart both devices
Sign out of main icloud account on mac
Restart mac
Sign into main icloud account on mac
Restart mac
Sign out of main icloud account on iphone
Restart iphone
Sign into main icloud account on iphone
Restart iphone
Check wifi on mac, profit


In my OS X console, See confirmation of Handoff:

AppleIDAuthAgent[285]: SERVER AppleIDAuth: Adding certificate for "&lt;my-apple-id&gt;@?????.???", "&lt;certificate number&gt;" to keychain, serial#="&lt;iOS device serial number&gt;" (updateAccountWithCertificateResult()/AppleIDAuthenticationImpl.cpp #225) accountCheckDispatch

Note that correct Instant Hotspot functionality is as follows


Instant hotspot should show up even if "Personal Hotspot" is turned off on the iPhone
Instant hotspot should show up even if Mac and iPhone are connected to other Wifi networks
Instant hotspot should show up even if phone is locked and in your pocket
Instant hotspot should show up even if iPhone has never been paired to the mac before
Instant hotspot will not show up if bluetooth is turned off on either device