When the issue occurs, I suggest to look into these directories:
/etc/apache2/conf/plesk.conf.d/vhosts
/etc/nginx/plesk.conf.d/vhosts
whether a symbolic link to the domain configuration file of your domain exists.
Also, while the issue occurs, check the Linux processes whether a reconfiguration is going on, e.g.
# ps aux | grep mng
And also look into /var/www/vhosts/system/<your domain>/conf if the httpd.conf and nginx.conf files for the domain exist.