Sorry I haven’t replied until now. I am convinced the portfolio plug-in was the problem. I’ve been working on the site now for a few days without any of the issue I had experienced when it was installed.
For starters, whenever I tried to install that plug-in, it seemed to take forever to install. So perhaps it was not installing completely. Does it reside on your server, behind a firewall or something like that, maybe?
Then, afterward, I would eventually get a white screen. Nothing: not a 404 error … just nothing.
I tried starting over with a brand-new WP installation and reinstalled the theme, etc. At least once I’m pretty sure it installed because I was able to activate it. But eventually, I would get that white screen, where it is impossible to see the site.
I worked with my webhost and determined by trial and error that it was the Portfolio plug-in causing the issue. We figured this out by disabling the plug-ins one by one.
I was running a basic set of plug-ins, which I am listing just in case you can determine if there is a conflict: bbPress, Revolution Slider, UpdraftPlus-Backup/Restore, WPML Multilingual CMS, WPML String Translation. I also had the maintenance mode plug-in that you suggest installing, but I cannot recall its name; I’m not using it now, so I’ve uninstalled it.
I don’t need the portfolio features at this moment, but I might like to use the plug-in in the future. Regardless of what I do, for your future clients, you should determine what is wrong.
Maybe if you install that set of plug-ins and then add the portfolio plug-in on top, you’ll be able to figure out what is happening.
Good luck and let me know if you have any further questions. And I’ll be curious to know what you figure out when you do.