I am running CiviCRM 5.17.4 on Wordpress 5.2.3. The problem I am facing is that I am unable to send mailing to mailing lists (defined as groups) via CiviMail.
When I try to enter the group in the Recipients box I get Loading Failed message and none of the available groups is loaded (see image below). The problem appears for existing and new mailings.
Interestingly when I re-use the existing mailing I can still send it to a previously used group (though I cannot change the recipient lists). The groups load normally in manage group page.
Any advice on the problem or debugging suggestions would be very welcomed.