Select Help -> Debug Log Settings...
Add the following line (note the leading # symbol)
#com.intellij.util.indexing:trace
Relaunch the IDE (don't need to invalidate cache as that will cause it to start from scratch, whereas restarting from the point of failure, for me anyway, reported the problem file as soon as I restarted):
Scheduling indexing of file://C:/dev/tools/ruby/lib/ruby/2.2.0/x64-mingw32/win32ole.so by request of index Stubs
Our project doesn't use win32ole so I moved the file to a safe location and restarted my IDE... Bingo, problem gone, indexing finally completed after almost 1 year of effectively using intellij as a slightly-smarter-than-notepad ruby editor.
