Hi, is it possible to install seafile on a shared webhost with mysql support or do I need a root server?
You would need a own root or vserver because you need to install packages on your server wich a Webhoster might not have installed.
See also: https://forum.seafile.de/t/install-seafile-on-shared-hosting-server/1049
Using a root server is always recommended. Some users got various problems which are hard to fix because of the limitations imposed on shared web hosts.