I just set up a test page. Completely empty, just installed Download Manager.
As soon as I activate the plugin, I can’t edit PHP files with the inbuild editor of wordpress anymore.
Unable to communicate back with site to check for fatal errors, so the PHP change was reverted. You will need to upload your PHP file change by some other means, such as by using SFTP.
Since I have this problem on two other sites, I think you should be able to replicate it.
