@mmaridev @ftimmers I found that epel provides a working guacd 1.5.2-2 in the epel-testing repo.
After executing
yum update guacd --enablerepo=epel-testing
Guacamole should work again.
Please also test the current Guacamole 1.5.2 (release notes):
yum install https://mrmarkuz.dynu.net/mirror/devtest/nethserver-guacamole-0.0.1-9.ns7.noarch.rpm