Horde does not load images. From logs it is shown that system tries to open image from wrong directory:
/usr/share/psa-horde/themes/default/graphics/ system tries to find image under domain directory
# less /var/log/nginx/error.log
[error] 14482#0: *232398 open() "/var/www/vhosts/domain.tld/httpdocs/themes/default/graphics/horde-power1.png" failed (2: No such file or directory), request: "GET /themes/default/graphics/horde-power1.png HTTP/1.1", upstream: "http://127.0.0.1:7080/themes/default/graphics/horde-power1.png", host: "webmail.domain.tld"
Nginx configuration is broken, try to disable and enable it again: