Plugins no longer working on https

Hi, this is my first post. I need some advice please. All of a sudden our WordPress website is not working as it was. It took me a while to figure out why some people see the correct site and others not.
When loading a page eg. /buffalo-rally/2019-2/ it loads fine. However when loading /buffalo-rally/2019-2/ it breaks and the tab plugin doesn’t work. Other plugins behave similar.

I don’t understand why it suddenly behaves this way? We have never experienced this before. I am a little nervous to load a https plugin.

Any advice will be really appreciated.

Thank you
Kevin
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