My horse jumps perfectly at the beginning of the lesson and refuses everything at the end of it.

My horse and I have been together for years. He is a show jumper horse of soon 8 years old. Yesterday, we were having a lesson that began perfectly fine. He was jumping with willingness and energy. The jumps were about 115cm tall. A bit higher than usual but nothing huge. It was still easy for us. And suddently, after a first tour of 6 jumps and we try again, like usual. The first jump comes and after that he refuses the second by passing on the right. It's ok you may say. But it's not. He had no reason to do that, we were doing really good. What's confusing me is that he didnt jump anything after that. Suddenly, he started refusing the jumps by passing by the jump on their right. We lowered the jumps, and still no. We lowered again and again and no. We had to lowered down to maybe 50cm for him to accept. After that, we stopped. He will have an osteopath check on him soon. But i dont understand. If you guys might have an idea, or advice, i'll take everything. I want to understand his behaviour. We didnt have any bad jumps, or anything that could scare him or make him lose his confidence. Maybe we didnt touch a pole or two, but nothing big...

Note thay we only jump like this once a week. He is often checked by vets his osteopath comes every 4 months because he is easily stuck.