Plugins Page – “Page disabled by the administrator”

I’ve actually just fixed it myself.

The problem was as follows:

I had changed the username of the admin account to ‘new-admin’. What I had missed when making this change was to update the ‘wp_sitemeta’ table in the database. meta_id 8 (site_admins) still had the old admin name set. Once I updated this, the plugins menu reappeared!