while you are waiting for one of my colleague to take over the ticket, can you please enable error logging that will save all errors in a file called debug.log, log file inside the /wp-content/ directory.
To do so, please add the following to your wp-config.php
Once you have added those lines to your wp-config.php file, please try replicate the issue. Then go to your installation's wp-content folder and find the "debug.log" file that should've appeared by now (it will appear if it encounters an error in your server).
Then, upload that file into a storage platform of your choosing (Google Drive, Dropbox, etc) and send me the link (make sure it's set as "public" otherwise i won't be able to access its content) so i can download it and take a look at it, and determine where to go from there.