It turns out this is because of my htaccess file.

In my htaccess I had the following lines to stop anyone accessing my WordPress Admin if they were outside of my IP address:

<LIMIT GET>
Order deny,allow
Deny from all
Allow 100.8.8.8
</LIMIT>

This stops anyone assessing myWordPress admin unless they are logging in from my IP address. Apparently this is now updated and to do this you now use the following command:

<LIMIT GET>
Require IP 100.8.8.8
</LIMIT>

I changed this and no more server 500 error!

I’ve just typed this up in case anyone else has the same problem. My new htaccess file sitting in my wordpress/wp-admin folder looks like this:

AuthUserFile /dev/null
 AuthGroupFile /dev/null
 AuthName "Example Access Control"
 AuthType Basic
 Require ip 220.233.28.8 

via Override WordPress Default Attachment “Link to” Setting.