I’d first make sure that everything is updated: WP, themes, plugins.
Then I’d look at the plugin’s WP page and see how long it’s been since they have been updated. You might have some old plugins that could be causing problems, like maybe they are using old JS.
I’d also look at the page source of a couple pages to see if there is anything unexpected (code) in there.
Since the problem is pagination, perhaps your theme is old, and doesn’t support new features. Take a look at the error.log file and see if there are errors relating to your theme (or any plugins).
And, perhaps, re-doing the Permalinks page (change it to something else, save it, then change it back to your preference) will ‘reset’ something.
Whatever it is, report back here so that your solution will help others.