This was unexpected and I am still not sure if I made aconfiguration error or if this is an unflexibility in HTML5 File Upload.
The server/directory structure differs from my local server to the remote server. Icould mange the setup with database connection by copying databse names / passwords etc to my local configuration. This is already quite annoying because on the rmote site you sometimes get some automated names which are hard to remember. And we don't have all clients on just one remote server.
What is much more annoying is the fact that H5FU ALWAYS seem to use absolute paths. This leads to a lot of inconsistencies. A working page (for inserting data including files for example) works on the local site but does not work on the remote site, because the absolute structure differs from the local to the remote.
Only when I set up a virtual host for this specific client on the server in the local LAN I could make it working on both destinations.
Guys, this schould definitely change, so that we have a choice. Its quite annoying to set-up a vhost for each client on the local server. HTML5 File Upload is the first and only extension I know so far which shows this inflexibility by only offering absolute paths. The more as the Server Connection Action Files are constantly uploaded to the remote server whenever there is a change or modification.

At the moment it looks to me as a typical developer's mistake: assuming that one's own setup can be setup worldwide. Internet servers definitely differ enormously in their approach (eg. public_html etc.)
Even Dreamweaver offers the choice between absolute / relative paths.
If I made a mistake or overlooked a configuration, please let me know. Maybe there is an easy option for this.

The extension is using an absolute path. On many of the servers parent paths are not allowed, due to security reasons.
You just need to make sure your DW site is properly setup - with the proper root folder selected, the same way as it is selected on your server. You cannot expect a script to work when your DW site is not properly setup.