Problems with Eclipse auto-compile and auto-completion

Question

I'm using Eclipse 4.7.2 with Scala IDE plugin 4.7.0. Most of my projects include Java and Scala code and are working fine.

I took over an existing project, also with Java and Scala code and noticed something strange - initially, the auto complete works just for the Scala code, not for the Java code. The existing Java code handling had several issues:

• The auto-complete doesn't work. When trying to use it I get an error message box saying "The 'Scala Trait Member Completions for Java' proposal computer from the 'org.scala-ide.sdt.core' plug-in did not complete normally", and then two more errors on the installed STS (Spring) auto-compete and the build in JDT auto complete.
• The auto-compile doesn't work. When adding new code that cannot be compiled, the Eclipse does not add an error.
• The code is not coloured properly: all white, only strings and keywords are coloured.

The strange thing is that when I add new Java classes, everything works well. These issues occur only on the old code.