Repeating “Unfortunately Google App has stopped” error on Nexus 5 My Nexus 5 stated acting strange in the past few days. 

Here's the short history: 


A few days ago, I was prompted to install a new version in OTA update. Done. 
Yesterday, phone shut off just like that by itself. When powered on, it has a message on a black screen saying "optimizing app x/150". When it was done, phone booted. 
This morning: wanted to access Google Now, but it wasn't there - I couldn't flick to the left. Was prompted to 'reactivate' google Now, which I did. I should mention it was there after the OTA update. 


Since then, every few seconds, and pretty much whatever I try to do, the error message in the title pops up. (side note: strangely, it says Google App and not Apps). Extremely hard to use the phone when every few seconds this pops up. Help, anyone? Thanks!
As it says "Google App", it's most likely the app named Google (no kidding: there's such one, just follow the link – though I cannot find it in Settings › Apps by that name). Just tried the one from the drawer, brings me to Google Now – which again matches the app you've got trouble with, as your question states: "wanted to access Google Now, but it wasn't there".

Going by the icon, the corresponding app in Settings must be Google Search – which again makes sense considering Google Now is part of that.

So I'd suggest you try to go there (in Settings › Apps) and clear it's cache. If that doesn't help, try clearing its data as well (same place). If that doesn't solve it, open the Google Settings app (not the general device settings, but the settings app with the "g" for "Google" in its icon – you can find it in the app drawer) and deactivate Now and Cards there. This disables Google Now, but solves the issue (as you've proven).

Hint in case you cannot reach out to Settings or the Google Settings app (e.g. if that pop-up reappears faster than you can tap somewhere else): You can clear all app caches at once by booting to recovery-mode and "wipe cache". No data lost that way. Another good hint in this context is safe-mode: if the troublemaker is a user-app, it won't be started here, so you can clear cache/delete data/deactivate/uninstall such an app from there.