Put the summary of the bug you're having here

I thougt that i could ride the horse, because it worked before.

But...

I just opened the horse-inventory

Steps to Reproduce:
1.Tame a horse. Equip the horse with a saddle and diamond armour
2. Sit up on the horse and ride it. Jup off after a couple of seconds.
3. Repeat step nr 2 5-10 times. Change gamemode 2-5 times ( both when you ride the horse and not).
4. Change gamemode to creative
5. Try to sit up on the horse. If it works as it did for me, do you only open the horse-inventory when you leftclick on the horse, you do not ride it.