Personal hotspot unavailable intermittently, "contact your carrier" A friend has an iPhone 6 and a plan that includes tethering.

A couple of weeks ago, her "personal hotspot" feature started acting up. It tends to be inactive. When you try to activate it, you get a message asking to contact the cellular provider. 

Turning the phone off and on again solves the problem.

The provider maintains that tethering is activated, and that there is no problem on their end. I checked the cellular data settings and they are OK.

What could be causing this?

Could it have been caused by a software update?  iOS is up to date.
Some cellular plans allow Personal Hotspot / Tethering, others do not. It's worth your friend checking with your cell provider whether it's allowed on the plan she's on and/or if there's a usage limit (highly likely) and that limit has been reached.

For reference, my old cell plan had 1GB of Personal Hotspot usage allowed and watching a couple of movies on the laptop using Netflix got through that real fast, so I upgraded my plan.