Unfortunately, we have not found a solution either. The best we could do is create a redirect so that the UM password reset page redirects to WordPress’ native password reset form. No other ideas from Ultimate Member have been forthcoming.
The site doesn’t have any security or caching plugins installed. Our host tells us that the only cache affecting the site is PHP Opcache. Is that a potential culprit?
I tried a password reset after clearing Opcache and there wasn’t any difference.