I've been going around in circles.
Moved a site yesterday, all seemed OK. Went to install a plugin this evening, and got a fatal error (I had been getting the same error when transferring the site, and had to keep re-starting).
Read various threads, tried so many things I can't keep track, managed to get from 500 error to regular Craft internal server error.
Turns out all the plugins I had installed are now missing, which means the site won't load.
Move the plugin folders from old server to new, and get a 500 error again.
How can I make the CMS recognise the previously installed plugins?