It could be either A) the person you were/are boarding with downgraded from being a subbed member to a basic member and no longer have access to the boarding center or B) a bug.

I'm not too sure how point A) works as I never understood how boarding with others work as I simply ended up boarding my horses at my own stables. I have plenty of room for them and all of my horses are frozen, atm, due to irl stuff. I just had 3 of them get kicked due to a downgrade during this month, one recently and two earlier this month. I am still a Gold tier member, have stalls and I am still under the limit ((ex: 227 stalls. 220 horses)). I was notified of this being a thing. If you play on desktop, check the Bell looking icon for notifications all the way on the far right side of the toolbar up top and it might show you there your horses being kicked. If on mobile, tap onto the three short lines and scroll all way to bottom ((for me I cannot always scroll all way to bottom so this may happen to you)) and also tap onto the Bell icon to see notifications that way and see what it may say.

Nonetheless. I at least did a bug report since mine are always boarded at home and your situation is different. I'd try seeing if the player you boarded with downgraded or its a bug? If it is a bug, perhaps post a bug report and or add onto my own?

Thank you for your suggestions. I will look into it! 😊