In my case quitting the app and relaunching it helped.
I think that my problem was that I actually had IntelliJ IDEA already updated but not relaunched as:
- before quitting updater shown updates available to install (and the error message when trying to apply them),
- after relaunch updater told me that I already have the latest version.
My other theory is that I had issues as I use 2 user accounts on my Mac and I may have been hit by something similar to this bug. But both first and second run of the app was done by the same user this time, so this shouldn't be the case here.