I have the same problem, also on multisite.
When I uploaded the plugin, I couldn’t turn it on, I got message: “Plugin cannot be activated due to a critical error}”.
So I deleted it and installed again through the WP interface. This time I was able to activate the plugin but got only message:
“InstantSearch+ for WooCommerce is installed ?? Choose your settings “
I clicked on “Choose your settings” and it opened BLANK page:
https://woo.instantsearchplus.com/wc_dashboard?site_id=&authentication_key=&new_tab=1&v=1.2.15
When I hovered the cursor over the instant search icon on menu, it didn’t open any menu. When I clicked the icon it opened again the blank site.
Bottom line, the plugin doesn’t work.