Invoice for Client dropdown – add client issue

Hi there,

Getting an issue whereby the little Plus to add a client while creating an invoice doesn’t work. Click on it, but nothing changes.
https://l.njpc.it/2rIe4TT

Everything’s up to date, but this has been happening on the last version of WP too, and also on the last version of Sprout.

Any idea if there’s anything I can do to fix this?

Thanks,
Pete