Migrate modules – source database fields disabled

I must be doing something particularly daft here.

I’m trying to use the Migrate modules in Drupal 8.4.4. I’ve installed Drupal successfully on a Synology NAS, and it seems to work. When I go to domain.dom/upgrade it still seems to work, but the fields for entering the source database are grayed out, and have the disabled property set to “disabled”. Only the Database Password field is enabled.

I’ve also noticed that I cannot now disable the Migrate modules; the check boxes aren’t enabled.

There seems to be something seriously amiss here. I didn’t have these problems when I tried with a much earlier version of D8 some months ago.

Drupal version: 

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