Hi everyone, I was finally able to nail this issue(s) down and fix them. So in case someone is struggeling with the same problems here is my solution: The problem ist NOT caused by the apache-poi 3.17 library directly. Same goes for the JDK9 thing.
It looks like Eclipse 4.7.2 has some problems with JDK9 compiler levels in cause of overlapping errors (sorry don't know how to say this better in english). The actual problem was triggered through the update from v3.14 to 3.17 witch brought some API changes (especially all those migrations to the new XXXType ENUMS like HyperLink, BorderStyle, FillPatternType, etc.). My Project uses the apache POI library in two separate classes (SwingWorkers) to read and/or write Excel documents. One class was marked as contains errors by eclipse (that class from witch i took the screenshots for the StackOverflow Post). The Second class did not have any errors (in Eclipse). After migrating the Errors in that one class based on the guide in the Apache POI Changelogs those "cannot be resolved" import issues appeared. The second class still had zero errors! I found out about those additional errors in the second class only by compiling the stuff by hand on the command line instead of from within eclipse. After fixing those parts (migrating from HyperLink.LINK_DOCUMENT to HyperlinkType.DOCUMENT) the import issues in the other class disappeared! So it looks like eclipse somehow messed up the imports / accessibility of some classes of the library for the whole project because of a completly other issue in another class of the project. Never had this kind of problems before.... So thank you all for your time, hope this helps others with similar issues :). It was definitly NOT a direct problem of the apache poi 3.17 alongside with jdk9! Greetings, Markus