Add Image, switch to Text and more fails after WP4.9 update

Hi, after updating to WP4.9 I noticed a bug with the insert link pop-up. Trying to fix it, I wanted to switch to text editor mode, but couldn’t. After searching for this issue I saw that this often coincides with Add Image problems, so I checked and that didnt work either.

My version is a Dutch one: WordPress 4.9–nl_NL_formal, and I use it with Spacious theme. This issue also showed up with updating to WordPress 4.9–en_US. I’ve tried disabling plug-ins and switching to standard theme, which didn’t help. I normally use Opera, but the same happened in Edge.
I tried to update twice, once through the updater of my host SiteGround, and once clicking update on the Dashboard. When I roll back to before the update, things are fine again.

I read about direct installation as a next step, but I’m not very familiar yet with my hosts back-end, so if anyone has any other ideas, or recognizes this problem, any thoughts would be appreciated.