Exact same problem – Details for newbies: There is a plugin Companion Auto Update. It normally works very well. With this bug, it allowed me to login, but after login threw up an error. If you go to a page and then EDIT that page, and then go into plugins, disable CAU. To be safe, after that log out, and then back in. Mine is operating perfectly after the disable. I like CAU, so didn’t delete it. I am hoping this bug is fixed in the next month or so.
WordPress 5.5.3 running Twenty Sixteen theme.
