The HTTP ERROR 500 - that’s from your site, correct? That may indicate something going on on your server - could you check your PHP error log and report back on any errors you see there?
If you are experiencing a new issue, can you please create a new post outlining the issue and explaining it in more detail (including what version you’re upgrading from and to)? We typically try to limit it to one post per issue, as per our guidelines, so let’s keep the topic of this post to the Crossref issue.
I solved this problem on my own by unticking/uninstalling Crossref Reference Linking plugin. When you install this plugin you may face difficulty in downloading CrossRef XML Export file.