My theme developer helped me out. WordPress v5.5 no longer enables a tool called Jquery-migrate. This not only affects the slider, but messes up the site in many other ways. I’m using an old theme, so I guess it’s depending on code that’s now depreciated.
Fortunately, there’s a plug-in that fixes this problem (for now at least). It’s called “Enable Jquery-Migrate”. Here’s the WP article explaining this: https://www.ads-software.com/plugins/enable-jquery-migrate-helper/#description
I installed this and it fixed my site. So far so good… At least it buys me some time before I take on the task of moving into a new theme.
