Maszek

I’ve called my webhost and asked him if he knew the solution to this problem. He confirmed it’s a known WordPress issue, but provided me with a workaround. As a webmaster, you should have access to a service center where you can change server settings. The service center is called Plesk server or Cosmos or whatever. Somewhere in your configuration panel, probably under hosting performance, there should be a possibility to change the compatibility mode. It’s a binary setting; so it’s either on or off. Turn it ‘on‘ when you want to install your updates. After doing so, turn it ‘off‘ again, since your site might be a bit more vulnerable.

@ Justin Greer

I would not rule out there is something wrong with 4.4 but considering millions of sites updated in under 24 hours and only a handful of issues arise with this problem, we have to look at something outside of 4.4.

The question is how many WordPress users will take the trouble to login to this forum and report this issue. There’s quite a threshold doing so, hence, the number of posts here may not be representative at all.
I’m experiencing the same plugin-problems after upgrading to WP 4.4. Trying the solutions proposed here is way above my competence and availability. After all, I decided to use WordPress assuming I would have smooth, bug-free CMS to maintain my websites, and not having to deal with tech issues.
(I’m sorry if this is no contribution to the solution, but I feel I had to share my frustration just as well)