500 Internal Server Error when accessing admin

I just get a server error when trying to access /admin, or any other URL. This is a completely new installation on a staging server. The ‘Welcome’ page shows OK.

I’m guessing this is a .htaccess problem, because if I remove it and change ‘omitScriptNameInUrls’ to false the login page shows – but unstyled. If I login I get 400 bad request.

I’ve looked in the web.log, but can’t see anything to help there.

The hosting company is IONOS (1&1).

Any ideas?