Even simplexml plugin looks enabled when I phpinfo() or php -m when I install and click on the “Edit with texture” button I get an internal server error.
I’m still unsure why it happens (I mean, looks like is disabled by default in some OS releases but it didn’t explain why it’s listed as enabled in my case), anyway the solution is fairly trivial:
$ sudo apt-get install php7.0-xml
$ apk add php7-xml php7-simplexml
I found this in the last ojs docker image, so I will PR now to get it fixed.