I just switched theme in admin to legacy and private message is now working. Must be a Nouveau bug.
I cannot duplicate your issue, but I do not have ‘storefront’ theme.
You could try narrowing it down by using Nouveau and then momentarily switch to a WP theme like 2016
If the issue persists, try:
– check your browser console
– check your error_logs
– turning off any minify code re javascript
– deactivating other plugins ( woocommerce ? )
And let us know what you find.