freeze with 'outdated stub'

Since a couple of days IntelliJ freezes with a 'outdated stub' error for one particular Ruby module when I use Cmd-X to remove text. I have to force stop it via Activity Monitor. This can be replicated again and again.

I removed the text with a external editor and put it back at another place in the code. Then I entered IntelliJ again and the text was moved, as expected. I could use Cmd-X again for other blocks of code, without freeze. But when I tried to move the first code block again, IntelliJ froze again.

What is this stub stuff about? 

Part of log:

2018-05-01 09:02:53,082 [11444829] ERROR - j.psi.stubs.StubTreeLoaderImpl - Outdated stub in index: file:///Users/Zorgverband/Workspace/zorgverband/app/controllers/reports_controller.rb indexed at 1525155409000 with document size 9147, doc=DocumentImpl[file:///Users/Zorgverband/Workspace/zorgverband/app/controllers/reports_controller.rb], docSaved=false, wasIndexedAlready=true, queried at 1525155409000
indexed lengths={chars=9147, bytes=9147}
doc length=6727
file length=9147
cached PSI class org.jetbrains.plugins.ruby.ruby.lang.psi.impl.RFileImpl
PSI length=6727
projects with file: 1
java.lang.Exception
at com.intellij.psi.stubs.StubTreeLoaderImpl.diagnoseLengthMismatch(StubTreeLoaderImpl.java:175)
at com.intellij.psi.stubs.StubTreeLoaderImpl.checkLengthMatch(StubTreeLoaderImpl.java:145)

....

at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)
2018-05-01 09:02:53,083 [11444830] ERROR - j.psi.stubs.StubTreeLoaderImpl - IntelliJ IDEA 2018.1 Build #IU-181.4203.550
2018-05-01 09:02:53,083 [11444830] ERROR - j.psi.stubs.StubTreeLoaderImpl - JDK: 1.8.0_152-release
2018-05-01 09:02:53,084 [11444831] ERROR - j.psi.stubs.StubTreeLoaderImpl - VM: OpenJDK 64-Bit Server VM
2018-05-01 09:02:53,084 [11444831] ERROR - j.psi.stubs.StubTreeLoaderImpl - Vendor: JetBrains s.r.o
2018-05-01 09:02:53,084 [11444831] ERROR - j.psi.stubs.StubTreeLoaderImpl - OS: Mac OS X
2018-05-01 09:02:53,084 [11444831] ERROR - j.psi.stubs.StubTreeLoaderImpl - Last Action: EditorCut

IntelliJ IDEA 2018.1 (Ultimate Edition)
Build #IU-181.4203.550, built on March 26, 2018
JRE: 1.8.0_152-release-1024-b15 x86_64
JVM: OpenJDK 64-Bit Server VM by JetBrains s.r.o
Mac OS X 10.11.6