Thanks, but that's not it. As i wrote, it works with FMP 11 from the same computer, but not with FMP 10. It's a FMS I'm trying to connect to...
Since FMP 11 works, why bother with version 10?
It might be an issue with the .pem file.
For More Information see: FMP installer does not install server.pem file (Windows)
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip