The error unable to coneect to server appears with the following steps only with Onweb Designer. Other applications like "App Express", "Object Builder", "Administrator" are working fine.

1. Click on OnWeb Designer Icon
2. the logon page is prompted
3. Enter logon details (Default profile; IP:  127.0.0.1, port 8084, user name 'admin, password - default password)
4. Enter "logon&Save" button
5. An error stating "Unable to connect to server" appears

Java 1.7 was the culprit here as Onweb 7.5 has not been tested yet with that version but somehow even though 1.6 was also installed the Onweb server did not want to select the 1.6 java version. After uninstalling Java 1.7 all works fine again.