For Maven projects you can go like this:
Open the maven the tool window. In Intellij you can do this by right click on the project name and then click on "maven tool window".
Then click on the reimport all maven projects.
Now your project should be stable again
Intellij has a lot of interesting features and for that it makes use of cache. That sometimes makes problems like this. After invalidating caches and restarting IntelliJ the dependencies that were cached are removed and updated on the next compiling, and these errors were fixed.
The solution to that problem is quite simple, you just have to install maven (if you haven't done it already) and then compile the projects with e.g. "mvn eclipse:eclipse".
In the beginning, it takes maven approx. 30 minutes to download all dependencies and required files but then is able to compile it without any problems..
At the user's home directory (e.g. C:\Users\\ on a Windows machine, or /Users/ on a Mac) there is a hidden directory, called .m2. Within this directory there is a settings.xml file. It may be that this file is not existing, than just create a new text file and save it as settings.xml
Withing the XML file, the following notation allows to manually specify a mirror: see attached image.
The reference of the xml elements is:
mirrorOf: The server ID of the repository being mirrored (e.g "central")
name: A name of the mirror (optional, just for documentation purpose)
url: The url of the mirror
id: A freely selectable id of the mirror entry
Please check the attached file for an example.