After downloading the latest source from the code repository, the build seemed to be broken since it reported about 18 errors saying something like:
The type [X] is not accessible due to restriction on required library [Y]....where [X] and [Y] are of course placeholders for the class and jar library :) . After some (actually long) search on the web, one of my collegues found the "solution" for the problem. It was actually a wrong Eclipse configuration of the Java compiler. What has to be done is to set the "Forbidden reference (access rules)" under the "Deprecated and restricted API" from "Error" to "Warning".
In this way Eclipse will treat the problem as a warning instead of an error. I discussed that configuration with the RSSOwl author and he affirmed me that it is fine to have the configuration set to "warning".