Missing java project

I use database db with two java libraries, libA and libB. libA uses libB, so I added the project db.NSF.libB.javalib (which is the suggested project fix) to the project tab of the buildpath.The project is then built with no errors. So far so good.

However, after saving, closing and reopening the designer, the added project folder (the java project) is missing and I always get hundreds of errors due to linkage problems. What goes wrong here?