By default, even Global admins do not have permissions to perform such actions, as eDiscovery effectively allows access to everyone's data, which is an obvious concern. As an admin, you can add permissions, by using the steps you've detailed above.
The 500 error is unfortunate, as it doesn't really tell us much. Can you capture the network traffic when performing the request and check the detailed error message therein?
The UI effectively uses PowerShell to manage said permissions, so you might as well try adding them via PowerShell directly. To do so, connect to the SCC endpoint first: https://learn.microsoft.com/en-us/powershell/exchange/connect-to-scc-powershell?view=exchange-ps
Once connected, use the following cmdlet:
Add-RoleGroupMember "eDiscovery Manager" -Member user@domain.com
Remember that you will also have to add the user to the specific case afterwards. Alternatively, add him as eDiscovery admin instead:
Add-eDiscoveryCaseAdmin -User user@domain.com